{
    "mode": "man",
    "parameter": "perldebug",
    "section": "1",
    "url": "https://www.chedong.com/phpMan.php/man/perldebug/1/json",
    "generated": "2026-10-06T11:56:14Z",
    "sections": {
        "NAME": {
            "content": "perldebug - Perl debugging\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "First of all, have you tried using \"use strict;\" and \"use warnings;\"?\n\nIf you're new to the Perl debugger, you may prefer to read perldebtut, which is a tutorial\nintroduction to the debugger.\n\nIf you're looking for the nitty gritty details of how the debugger is implemented, you may\nprefer to read perldebguts.\n\nFor in-depth technical usage details, see perl5db.pl, the documentation of the debugger\nitself.\n",
            "subsections": []
        },
        "The Perl Debugger": {
            "content": "If you invoke Perl with the -d switch, your script runs under the Perl source debugger.  This\nworks like an interactive Perl environment, prompting for debugger commands that let you\nexamine source code, set breakpoints, get stack backtraces, change the values of variables,\netc.  This is so convenient that you often fire up the debugger all by itself just to test\nout Perl constructs interactively to see what they do.  For example:\n\n$ perl -d -e 42\n\nIn Perl, the debugger is not a separate program the way it usually is in the typical compiled\nenvironment.  Instead, the -d flag tells the compiler to insert source information into the\nparse trees it's about to hand off to the interpreter.  That means your code must first\ncompile correctly for the debugger to work on it.  Then when the interpreter starts up, it\npreloads a special Perl library file containing the debugger.\n\nThe program will halt right before the first run-time executable statement (but see below\nregarding compile-time statements) and ask you to enter a debugger command.  Contrary to\npopular expectations, whenever the debugger halts and shows you a line of code, it always\ndisplays the line it's about to execute, rather than the one it has just executed.\n\nAny command not recognized by the debugger is directly executed (\"eval\"'d) as Perl code in\nthe current package.  (The debugger uses the DB package for keeping its own state\ninformation.)\n\nNote that the said \"eval\" is bound by an implicit scope. As a result any newly introduced\nlexical variable or any modified capture buffer content is lost after the eval. The debugger\nis a nice environment to learn Perl, but if you interactively experiment using material which\nshould be in the same scope, stuff it in one line.\n\nFor any text entered at the debugger prompt, leading and trailing whitespace is first\nstripped before further processing.  If a debugger command coincides with some function in\nyour own program, merely precede the function with something that doesn't look like a\ndebugger command, such as a leading \";\" or perhaps a \"+\", or by wrapping it with parentheses\nor braces.\n",
            "subsections": [
                {
                    "name": "Calling the Debugger",
                    "content": "There are several ways to call the debugger:\n\nperl -d programname\nOn the given program identified by \"programname\".\n\nperl -d -e 0\nInteractively supply an arbitrary \"expression\" using \"-e\".\n\nperl -d:ptkdb programname\nDebug a given program via the Devel::ptkdb GUI.\n\nperl -dt threadedprogramname\nDebug a given program using threads (experimental).\n\nIf  Perl  is  called  with  the \"-d\" switch, the variable $^P will hold a true value. This is\nuseful if you need to know if your code is running under the debugger:\n\nif ( $^P ) {\n# running under the debugger\n}\n\nSee \"$^P\" in perlvar for more information on the variable.\n"
                },
                {
                    "name": "Debugger Commands",
                    "content": "The interactive debugger understands the following commands:\n\nh           Prints out a summary help message\n\nh [command] Prints out a help message for the given debugger command.\n\nh h         The special argument of \"h h\" produces the entire help page, which is quite long.\n\nIf the output of the \"h h\" command (or any command, for that matter) scrolls past\nyour screen, precede the command with a leading pipe  symbol  so  that  it's  run\nthrough your pager, as in\n\nDB> |h h\n\nYou may change the pager which is used via \"o pager=...\" command.\n\np expr      Same  as  \"print {$DB::OUT} expr\" in the current package.  In particular, because\nthis is just Perl's own \"print\" function, this means that nested data  structures\nand objects are not dumped, unlike with the \"x\" command.\n\nThe \"DB::OUT\" filehandle is opened to /dev/tty, regardless of where STDOUT may be\nredirected to.\n\nx [maxdepth] expr\nEvaluates  its  expression  in list context and dumps out the result in a pretty-\nprinted fashion.  Nested data structures are printed out recursively, unlike  the\nreal  \"print\"  function  in Perl.  When dumping hashes, you'll probably prefer 'x\n\\%h' rather than 'x %h'.  See Dumpvalue if you'd like to do this yourself.\n\nThe output format is governed by multiple options described  under  \"Configurable\nOptions\".\n\nIf the \"maxdepth\" is included, it must be a numeral N; the value is dumped only N\nlevels deep, as if the \"dumpDepth\" option had been temporarily set to N.\n\nV [pkg [vars]]\nDisplay  all  (or  some) variables in package (defaulting to \"main\") using a data\npretty-printer (hashes show their keys and values so you see what's what, control\ncharacters are made printable, etc.).  Make sure you don't put the type specifier\n(like \"$\") there, just the symbol names, like this:\n\nV DB filename line\n\nUse \"~pattern\" and \"!pattern\" for positive and negative regexes.\n\nThis is similar to calling the \"x\" command on each applicable var.\n\nX [vars]    Same as \"V currentpackage [vars]\".\n\ny [level [vars]]\nDisplay all (or some) lexical variables (mnemonic: \"mY\" variables) in the current\nscope or level scopes higher.  You can limit the variables that you see with vars\nwhich works exactly as it does for  the  \"V\"  and  \"X\"  commands.   Requires  the\nPadWalker  module  version  0.08  or  higher;  will warn if this isn't installed.\nOutput is pretty-printed in  the  same  style  as  for  \"V\"  and  the  format  is\ncontrolled by the same options.\n\nT           Produce a stack backtrace.  See below for details on its output.\n\ns [expr]    Single  step.  Executes until the beginning of another statement, descending into\nsubroutine calls.  If an expression is supplied that includes function calls,  it\ntoo will be single-stepped.\n\nn [expr]    Next.  Executes over subroutine calls, until the beginning of the next statement.\nIf  an  expression is supplied that includes function calls, those functions will\nbe executed with stops before each statement.\n\nr           Continue until the return from the current subroutine.  Dump the return value  if\nthe \"PrintRet\" option is set (default).\n\n<CR>        Repeat last \"n\" or \"s\" command.\n\nc [line|sub]\nContinue,  optionally  inserting a one-time-only breakpoint at the specified line\nor subroutine.\n\nl           List next window of lines.\n\nl min+incr  List \"incr+1\" lines starting at \"min\".\n\nl min-max   List lines \"min\" through \"max\".  \"l -\" is synonymous to \"-\".\n\nl line      List a single line.\n\nl subname   List first window of lines from subroutine.   subname  may  be  a  variable  that\ncontains a code reference.\n\n-           List previous window of lines.\n\nv [line]    View a few lines of code around the current line.\n\n.           Return  the  internal  debugger  pointer to the line last executed, and print out\nthat line.\n\nf filename  Switch to viewing a different file or \"eval\" statement.  If  filename  is  not  a\nfull pathname found in the values of %INC, it is considered a regex.\n\n\"eval\"ed  strings  (when accessible) are considered to be filenames: \"f (eval 7)\"\nand \"f eval 7\\b\" access the body of the 7th \"eval\"ed  string  (in  the  order  of\nexecution).   The bodies of the currently executed \"eval\" and of \"eval\"ed strings\nthat define subroutines are saved and thus accessible.\n\n/pattern/   Search forwards for pattern (a Perl regex); final / is optional.  The  search  is\ncase-insensitive by default.\n\n?pattern?   Search  backwards  for  pattern;  final  ?  is  optional.   The  search  is case-\ninsensitive by default.\n\nL [abw]     List (default all) actions, breakpoints and watch expressions\n\nS [[!]regex]\nList subroutine names [not] matching the regex.\n\nt [n]       Toggle trace mode (see also the \"AutoTrace\" option).  Optional  argument  is  the\nmaximum  number  of  levels  to trace below the current one; anything deeper than\nthat will be silent.\n\nt [n] expr  Trace through execution of \"expr\".  Optional first argument is the maximum number\nof levels to trace below the current one;  anything  deeper  than  that  will  be\nsilent.  See \"Frame Listing Output Examples\" in perldebguts for examples.\n\nb           Sets breakpoint on current line\n\nb [line] [condition]\nSet  a  breakpoint  before  the  given  line.   If a condition is specified, it's\nevaluated each time the statement is reached: a breakpoint is taken only  if  the\ncondition is true.  Breakpoints may only be set on lines that begin an executable\nstatement.  Conditions don't use \"if\":\n\nb 237 $x > 30\nb 237 ++$count237 < 11\nb 33 /pattern/i\n\nIf the line number is \".\", sets a breakpoint on the current line:\n\nb . $n > 100\n\nb [file]:[line] [condition]\nSet  a  breakpoint  before  the  given line in a (possibly different) file.  If a\ncondition is specified, it's evaluated each time  the  statement  is  reached:  a\nbreakpoint  is  taken only if the condition is true.  Breakpoints may only be set\non lines that begin an executable statement.  Conditions don't use \"if\":\n\nb lib/MyModule.pm:237 $x > 30\nb /usr/lib/perl5/siteperl/CGI.pm:100 ++$count100 < 11\n\nb subname [condition]\nSet a breakpoint before the first line of the named subroutine.  subname may be a\nvariable containing a code reference (in this case condition is not supported).\n\nb postpone subname [condition]\nSet a breakpoint at first line of subroutine after it is compiled.\n\nb load filename\nSet a breakpoint before the first executed line of the filename, which should  be\na full pathname found amongst the %INC values.\n\nb compile subname\nSets  a  breakpoint  before  the  first  statement  executed  after the specified\nsubroutine is compiled.\n\nB line      Delete a breakpoint from the specified line.\n\nB *         Delete all installed breakpoints.\n\ndisable [file]:[line]\nDisable  the  breakpoint  so  it  won't  stop  the  execution  of  the   program.\nBreakpoints  are  enabled  by  default  and  can be re-enabled using the \"enable\"\ncommand.\n\ndisable [line]\nDisable  the  breakpoint  so  it  won't  stop  the  execution  of  the   program.\nBreakpoints  are  enabled  by  default  and  can be re-enabled using the \"enable\"\ncommand.\n\nThis is done for a breakpoint in the current file.\n\nenable [file]:[line]\nEnable the breakpoint so it will stop the execution of the program.\n\nenable [line]\nEnable the breakpoint so it will stop the execution of the program.\n\nThis is done for a breakpoint in the current file.\n\na [line] command\nSet an action to be done before the line is executed.  If line is omitted, set an\naction on the line about to be executed.  The sequence  of  steps  taken  by  the\ndebugger is\n\n1. check for a breakpoint at this line\n2. print the line if necessary (tracing)\n3. do any actions associated with that line\n4. prompt user if at a breakpoint or in single-step\n5. evaluate line\n\nFor example, this will print out $foo every time line 53 is passed:\n\na 53 print \"DB FOUND $foo\\n\"\n\nA line      Delete an action from the specified line.\n\nA *         Delete all installed actions.\n\nw expr      Add  a  global  watch-expression.  Whenever a watched global changes the debugger\nwill stop and display the old and new values.\n\nW expr      Delete watch-expression\n\nW *         Delete all watch-expressions.\n\no           Display all options.\n\no booloption ...\nSet each listed Boolean option to the value 1.\n\no anyoption? ...\nPrint out the value of one or more options.\n\no option=value ...\nSet the value of one or more options.  If the value has internal  whitespace,  it\nshould be quoted.  For example, you could set \"o pager=\"less -MQeicsNfr\"\" to call\nless  with  those  specific options.  You may use either single or double quotes,\nbut if you do, you must escape any embedded instances of same sort of  quote  you\nbegan  with,  as  well  as any escaping any escapes that immediately precede that\nquote but which are not meant to escape the quote itself.  In  other  words,  you\nfollow single-quoting rules irrespective of the quote; eg: \"o option='this isn\\'t\nbad'\" or \"o option=\"She said, \\\"Isn't it?\\\"\"\".\n\nFor historical reasons, the \"=value\" is optional, but defaults to 1 only where it\nis  safe  to  do so--that is, mostly for Boolean options.  It is always better to\nassign a specific value using \"=\".  The \"option\"  can  be  abbreviated,  but  for\nclarity  probably  should  not  be.   Several  options  can be set together.  See\n\"Configurable Options\" for a list of these.\n\n< ?         List out all pre-prompt Perl command actions.\n\n< [ command ]\nSet an action (Perl command) to happen before every debugger  prompt.   A  multi-\nline command may be entered by backslashing the newlines.\n\n< *         Delete all pre-prompt Perl command actions.\n\n<< command  Add  an  action  (Perl command) to happen before every debugger prompt.  A multi-\nline command may be entered by backwhacking the newlines.\n\n> ?         List out post-prompt Perl command actions.\n\n> command   Set an action (Perl command) to happen after the prompt when you've just given  a\ncommand  to  return to executing the script.  A multi-line command may be entered\nby backslashing the newlines (we bet you couldn't have guessed this by now).\n\n> *         Delete all post-prompt Perl command actions.\n\n>> command  Adds an action (Perl command) to happen after the prompt when you've just given a\ncommand to return to executing the script.  A multi-line command may  be  entered\nby backslashing the newlines.\n\n{ ?         List out pre-prompt debugger commands.\n\n{ [ command ]\nSet  an  action  (debugger  command)  to  happen before every debugger prompt.  A\nmulti-line command may be entered in the customary fashion.\n\nBecause this command is in some senses new, a warning is issued if you appear  to\nhave  accidentally entered a block instead.  If that's what you mean to do, write\nit as with \";{ ... }\" or even \"do { ... }\".\n\n{ *         Delete all pre-prompt debugger commands.\n\n{{ command  Add an action (debugger command) to  happen  before  every  debugger  prompt.   A\nmulti-line command may be entered, if you can guess how: see above.\n\n! number    Redo a previous command (defaults to the previous command).\n\n! -number   Redo number'th previous command.\n\n! pattern   Redo last command that started with pattern.  See \"o recallCommand\", too.\n\n!! cmd      Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See \"o shellBang\",\nalso.  Note that the user's current shell (well, their $ENV{SHELL} variable) will\nbe  used, which can interfere with proper interpretation of exit status or signal\nand coredump information.\n\nsource file Read and execute debugger commands from file.  file may itself  contain  \"source\"\ncommands.\n\nH -number   Display last n commands.  Only commands longer than one character are listed.  If\nnumber is omitted, list them all.\n\nq or ^D     Quit.   (\"quit\"  doesn't  work for this, unless you've made an alias) This is the\nonly supported way to exit the debugger, though typing \"exit\" twice might work.\n\nSet the \"inhibitexit\" option to 0 if you want to be able to step off the end the\nscript.  You may also need to set $finished to 0 if  you  want  to  step  through\nglobal destruction.\n\nR           Restart the debugger by exec()ing a new session.  We try to maintain your history\nacross this, but internal settings and command-line options may be lost.\n\nThe  following  setting  are  currently preserved: history, breakpoints, actions,\ndebugger options, and the Perl command-line options -w, -I, and -e.\n\n|dbcmd      Run the debugger command, piping DB::OUT into your current pager.\n\n||dbcmd     Same as \"|dbcmd\" but DB::OUT is temporarily \"select\"ed as well.\n\n= [alias value]\nDefine a command alias, like\n\n= quit q\n\nor list current aliases.\n\ncommand     Execute command as a Perl statement.  A trailing semicolon will be supplied.   If\nthe Perl statement would otherwise be confused for a Perl debugger, use a leading\nsemicolon, too.\n\nm expr      List  which methods may be called on the result of the evaluated expression.  The\nexpression may evaluated to a reference to a blessed  object,  or  to  a  package\nname.\n\nM           Display all loaded modules and their versions.\n\nman [manpage]\nDespite  its  name,  this calls your system's default documentation viewer on the\ngiven page, or on the viewer itself if manpage is omitted.   If  that  viewer  is\nman,  the  current  \"Config\"  information  is used to invoke man using the proper\nMANPATH or -M manpath option.  Failed lookups of the form \"XXX\" that match  known\nmanpages  of the form perlXXX will be retried.  This lets you type \"man debug\" or\n\"man op\" from the debugger.\n\nOn systems traditionally bereft of a usable man  command,  the  debugger  invokes\nperldoc.   Occasionally  this  determination  is  incorrect  due  to recalcitrant\nvendors or rather more felicitously, to enterprising users.   If  you  fall  into\neither category, just manually set the $DB::doccmd variable to whatever viewer to\nview  the  Perl  documentation on your system.  This may be set in an rc file, or\nthrough direct  assignment.   We're  still  waiting  for  a  working  example  of\nsomething along the lines of:\n\n$DB::doccmd = 'netscape -remote http://something.here/';\n"
                },
                {
                    "name": "Configurable Options",
                    "content": "The  debugger  has  numerous  options settable using the \"o\" command, either interactively or\nfrom the environment or an rc file. The file is named ./.perldb or ~/.perldb under Unix  with\n/dev/tty, perldb.ini otherwise.\n\n\"recallCommand\", \"ShellBang\"\nThe  characters  used to recall a command or spawn a shell.  By default, both are\nset to \"!\", which is unfortunate.\n\n\"pager\"     Program to use for output of pager-piped commands (those  beginning  with  a  \"|\"\ncharacter.)   By  default,  $ENV{PAGER}  will be used.  Because the debugger uses\nyour current terminal characteristics for bold and  underlining,  if  the  chosen\npager  does  not  pass  escape  sequences  through  unchanged, the output of some\ndebugger commands will not be readable when sent through the pager.\n\n\"tkRunning\" Run Tk while prompting (with ReadLine).\n\n\"signalLevel\", \"warnLevel\", \"dieLevel\"\nLevel of verbosity.  By default, the debugger leaves your exceptions and warnings\nalone, because altering them can  break  correctly  running  programs.   It  will\nattempt  to print a message when uncaught INT, BUS, or SEGV signals arrive.  (But\nsee the mention of signals in \"BUGS\" below.)\n\nTo disable this default safe mode, set these values to something higher  than  0.\nAt  a  level of 1, you get backtraces upon receiving any kind of warning (this is\noften annoying) or  exception  (this  is  often  valuable).   Unfortunately,  the\ndebugger  cannot  discern fatal exceptions from non-fatal ones.  If \"dieLevel\" is\neven 1, then your  non-fatal  exceptions  are  also  traced  and  unceremoniously\naltered  if  they  came  from \"eval'ed\" strings or from any kind of \"eval\" within\nmodules you're attempting to load.  If \"dieLevel\" is 2, the debugger doesn't care\nwhere they came from:  It usurps your exception handler and prints out  a  trace,\nthen  modifies  all  exceptions with its own embellishments.  This may perhaps be\nuseful for some tracing purposes, but tends to  hopelessly  destroy  any  program\nthat takes its exception handling seriously.\n\n\"AutoTrace\" Trace mode (similar to \"t\" command, but can be put into \"PERLDBOPTS\").\n\n\"LineInfo\"  File   or   pipe  to  print  line  number  info  to.   If  it  is  a  pipe  (say,\n\"|visualperldb\"), then a short message is used.  This is the mechanism used  to\ninteract  with  a  client  editor or visual debugger, such as the special \"vi\" or\n\"emacs\" hooks, or the \"ddd\" graphical debugger.\n\n\"inhibitexit\"\nIf 0, allows stepping off the end of the script.\n\n\"PrintRet\"  Print return value after \"r\" command if set (default).\n\n\"ornaments\" Affects screen appearance of the command line  (see  Term::ReadLine).   There  is\ncurrently no way to disable these, which can render some output illegible on some\ndisplays, or with some pagers.  This is considered a bug.\n\n\"frame\"     Affects the printing of messages upon entry and exit from subroutines.  If \"frame\n&  2\"  is  false,  messages are printed on entry only. (Printing on exit might be\nuseful if interspersed with other messages.)\n\nIf \"frame & 4\", arguments to functions are printed, plus context and caller info.\nIf \"frame & 8\", overloaded \"stringify\" and  \"tie\"d  \"FETCH\"  is  enabled  on  the\nprinted  arguments.   If  \"frame  &  16\", the return value from the subroutine is\nprinted.\n\nThe length at which the argument list  is  truncated  is  governed  by  the  next\noption:\n\n\"maxTraceLen\"\nLength to truncate the argument list when the \"frame\" option's bit 4 is set.\n\n\"windowSize\"\nChange the size of code list window (default is 10 lines).\n\nThe following options affect what happens with \"V\", \"X\", and \"x\" commands:\n\n\"arrayDepth\", \"hashDepth\"\nPrint only first N elements ('' for all).\n\n\"dumpDepth\" Limit  recursion  depth to N levels when dumping structures.  Negative values are\ninterpreted as infinity.  Default: infinity.\n\n\"compactDump\", \"veryCompact\"\nChange the style of array and hash output.  If \"compactDump\", short array may  be\nprinted on one line.\n\n\"globPrint\" Whether to print contents of globs.\n\n\"DumpDBFiles\"\nDump arrays holding debugged files.\n\n\"DumpPackages\"\nDump symbol tables of packages.\n\n\"DumpReused\"\nDump contents of \"reused\" addresses.\n\n\"quote\", \"HighBit\", \"undefPrint\"\nChange  the  style  of string dump.  The default value for \"quote\" is \"auto\"; one\ncan enable double-quotish or single-quotish format by setting it to \"\"\"  or  \"'\",\nrespectively.   By  default,  characters  with  their  high  bit  set are printed\nverbatim.\n\n\"UsageOnly\" Rudimentary per-package memory usage dump.   Calculates  total  size  of  strings\nfound  in variables in the package.  This does not include lexicals in a module's\nfile scope, or lost in closures.\n\n\"HistFile\"  The path of the file from which the history  (assuming  a  usable  Term::ReadLine\nbackend) will be read on the debugger's startup, and to which it will be saved on\nshutdown  (for  persistence  across  sessions).  Similar  in  concept  to  Bash's\n\".bashhistory\" file.\n\n\"HistSize\"  The count of the saved lines in the history (assuming \"HistFile\" above).\n\nAfter the rc file is read, the debugger reads the $ENV{PERLDBOPTS} environment variable  and\nparses  this  as  the remainder of a \"O ...\"  line as one might enter at the debugger prompt.\nYou may place the initialization options \"TTY\", \"noTTY\", \"ReadLine\", and \"NonStop\" there.\n\nIf your rc file contains:\n\nparseoptions(\"NonStop=1 LineInfo=db.out AutoTrace\");\n\nthen your script will run without human intervention, putting trace information into the file\ndb.out.  (If you interrupt it, you'd better reset \"LineInfo\" to /dev/tty if you expect to see\nanything.)\n\n\"TTY\"       The TTY to use for debugging I/O.\n\n\"noTTY\"     If set, the debugger goes into \"NonStop\" mode and will not connect to a TTY.   If\ninterrupted  (or  if  control  goes  to  the  debugger  via  explicit  setting of\n$DB::signal or $DB::single from the Perl script), it connects to a TTY  specified\nin  the  \"TTY\"  option  at  startup,  or  to  a  tty  found  at runtime using the\n\"Term::Rendezvous\" module of your choice.\n\nThis module should implement a method named \"new\" that returns an object with two\nmethods: \"IN\" and \"OUT\".  These should return filehandles to  use  for  debugging\ninput  and  output  correspondingly.  The \"new\" method should inspect an argument\ncontaining    the    value    of    $ENV{PERLDBNOTTY}     at     startup,     or\n\"$ENV{HOME}/.perldbtty$$\"  otherwise.   This  file  is  not  inspected for proper\nownership, so security hazards are theoretically possible.\n\n\"ReadLine\"  If false, readline support  in  the  debugger  is  disabled  in  order  to  debug\napplications that themselves use ReadLine.\n\n\"NonStop\"   If  set,  the  debugger  goes  into  non-interactive  mode  until interrupted, or\nprogrammatically by setting $DB::signal or $DB::single.\n\nHere's an example of using the $ENV{PERLDBOPTS} variable:\n\n$ PERLDBOPTS=\"NonStop frame=2\" perl -d myprogram\n\nThat will run the script myprogram without human intervention, printing  out  the  call  tree\nwith entry and exit points.  Note that \"NonStop=1 frame=2\" is equivalent to \"N f=2\", and that\noriginally,  options  could  be  uniquely abbreviated by the first letter (modulo the \"Dump*\"\noptions).  It is nevertheless recommended  that  you  always  spell  them  out  in  full  for\nlegibility and future compatibility.\n\nOther examples include\n\n$ PERLDBOPTS=\"NonStop LineInfo=listing frame=2\" perl -d myprogram\n\nwhich  runs  script non-interactively, printing info on each entry into a subroutine and each\nexecuted line into the file named listing.  (If you interrupt  it,  you  would  better  reset\n\"LineInfo\" to something \"interactive\"!)\n\nOther examples include (using standard shell syntax to show environment variable settings):\n\n$ ( PERLDBOPTS=\"NonStop frame=1 AutoTrace LineInfo=tperl.out\"\nperl -d myprogram )\n\nwhich  may  be useful for debugging a program that uses Term::ReadLine itself.  Do not forget\nto detach your shell from the TTY in the window  that  corresponds  to  /dev/ttyXX,  say,  by\nissuing a command like\n\n$ sleep 1000000\n\nSee \"Debugger Internals\" in perldebguts for details.\n"
                },
                {
                    "name": "Debugger Input/Output",
                    "content": "Prompt  The debugger prompt is something like\n\nDB<8>\n\nor even\n\nDB<<17>>\n\nwhere  that  number  is  the  command  number, and which you'd use to access with the\nbuilt-in csh-like history mechanism.  For example, \"!17\" would repeat command  number\n17.   The  depth  of  the angle brackets indicates the nesting depth of the debugger.\nYou could get more than one set of brackets, for  example,  if  you'd  already  at  a\nbreakpoint  and  then  printed  the  result  of  a  function  call  that itself has a\nbreakpoint, or you step into an expression via \"s/n/t expression\" command.\n\nMultiline commands\nIf you want to enter a multi-line command,  such  as  a  subroutine  definition  with\nseveral  statements  or  a  format,  escape  the  newline that would normally end the\ndebugger command with a backslash.  Here's an example:\n\nDB<1> for (1..4) {         \\\ncont:     print \"ok\\n\";   \\\ncont: }\nok\nok\nok\nok\n\nNote that this business of escaping a newline is  specific  to  interactive  commands\ntyped into the debugger.\n\nStack backtrace\nHere's an example of what a stack backtrace via \"T\" command might look like:\n\n$ = main::infested called from file 'Ambulation.pm' line 10\n@ = Ambulation::legs(1, 2, 3, 4) called from file 'camelflea'\nline 7\n$ = main::pests('bactrian', 4) called from file 'camelflea'\nline 4\n\nThe  left-hand  character  up  there  indicates the context in which the function was\ncalled, with \"$\" and \"@\" meaning  scalar  or  list  contexts  respectively,  and  \".\"\nmeaning void context (which is actually a sort of scalar context).  The display above\nsays  that you were in the function \"main::infested\" when you ran the stack dump, and\nthat it was called in scalar context from line 10  of  the  file  Ambulation.pm,  but\nwithout  any  arguments  at  all, meaning it was called as &infested.  The next stack\nframe shows that the function \"Ambulation::legs\" was called in list context from  the\ncamelflea  file  with four arguments.  The last stack frame shows that \"main::pests\"\nwas called in scalar context, also from camelflea, but from line 4.\n\nIf you execute the \"T\" command from inside an active \"use\" statement,  the  backtrace\nwill contain both a \"require\" frame and an \"eval\" frame.\n\nLine Listing Format\nThis shows the sorts of output the \"l\" command can produce:\n\nDB<<13>> l\n101:        @i{@i} = ();\n102:b       @isa{@i,$pack} = ()\n103             if(exists $i{$prevpack} || exists $isa{$pack});\n104     }\n105\n106     next\n107==>      if(exists $isa{$pack});\n108\n109:a   if ($extra-- > 0) {\n110:        %isa = ($pack,1);\n\nBreakable  lines  are  marked with \":\".  Lines with breakpoints are marked by \"b\" and\nthose with actions by \"a\".  The line that's about to be executed is marked by \"==>\".\n\nPlease be aware that code in debugger listings may not look the same as your original\nsource code.  Line directives and external source filters can alter the  code  before\nPerl  sees  it,  causing code to move from its original positions or take on entirely\ndifferent forms.\n\nFrame listing\nWhen the \"frame\" option is set, the debugger  would  print  entered  (and  optionally\nexited)  subroutines  in  different  styles.   See  perldebguts  for  incredibly long\nexamples of these.\n"
                },
                {
                    "name": "Debugging Compile-Time Statements",
                    "content": "If you have compile-time executable statements (such as  code  within  BEGIN,  UNITCHECK  and\nCHECK blocks or \"use\" statements), these will not be stopped by debugger, although \"require\"s\nand  INIT  blocks will, and compile-time statements can be traced with the \"AutoTrace\" option\nset in \"PERLDBOPTS\").  From your own Perl code, however, you can transfer  control  back  to\nthe debugger using the following statement, which is harmless if the debugger is not running:\n\n$DB::single = 1;\n\nIf  you set $DB::single to 2, it's equivalent to having just typed the \"n\" command, whereas a\nvalue of 1 means the \"s\" command.  The $DB::trace  variable should be set to  1  to  simulate\nhaving typed the \"t\" command.\n\nAnother way to debug compile-time code is to start the debugger, set a breakpoint on the load\nof some module:\n\nDB<7> b load f:/perllib/lib/Carp.pm\nWill stop on load of 'f:/perllib/lib/Carp.pm'.\n\nand  then  restart  the debugger using the \"R\" command (if possible).  One can use \"b compile\nsubname\" for the same purpose.\n"
                },
                {
                    "name": "Debugger Customization",
                    "content": "The debugger probably contains enough configuration hooks that you won't ever have to  modify\nit yourself.  You may change the behaviour of the debugger from within the debugger using its\n\"o\"  command,  from  the  command  line  via the \"PERLDBOPTS\" environment variable, and from\ncustomization files.\n\nYou can do some customization by setting up a .perldb  file,  which  contains  initialization\ncode.   For instance, you could make aliases like these (the last one is one people expect to\nbe there):\n\n$DB::alias{'len'}  = 's/^len(.*)/p length($1)/';\n$DB::alias{'stop'} = 's/^stop (at|in)/b/';\n$DB::alias{'ps'}   = 's/^ps\\b/p scalar /';\n$DB::alias{'quit'} = 's/^quit(\\s*)/exit/';\n\nYou can change options from .perldb by using calls like this one;\n\nparseoptions(\"NonStop=1 LineInfo=db.out AutoTrace=1 frame=2\");\n\nThe code is executed in the package \"DB\".  Note that .perldb is processed  before  processing\n\"PERLDBOPTS\".   If .perldb defines the subroutine \"afterinit\", that function is called after\ndebugger initialization ends.  .perldb may be contained in the current directory, or  in  the\nhome  directory.  Because this file is sourced in by Perl and may contain arbitrary commands,\nfor security reasons, it must be owned by the superuser or the current user, and writable  by\nno one but its owner.\n\nYou  can  mock  TTY  input  to  debugger  by adding arbitrary commands to @DB::typeahead. For\nexample, your .perldb file might contain:\n\nsub afterinit { push @DB::typeahead, \"b 4\", \"b 6\"; }\n\nWhich would attempt  to  set  breakpoints  on  lines  4  and  6  immediately  after  debugger\ninitialization.  Note  that  @DB::typeahead  is  not  a supported interface and is subject to\nchange in future releases.\n\nIf you want to modify the debugger, copy perl5db.pl from the Perl library to another name and\nhack it to your heart's content.  You'll then want to set your \"PERL5DB\" environment variable\nto say something like this:\n\nBEGIN { require \"myperl5db.pl\" }\n\nAs a last resort, you could also use \"PERL5DB\" to customize the debugger by directly  setting\ninternal variables or calling debugger functions.\n\nNote  that  any  variables  and  functions  that  are  not documented in this document (or in\nperldebguts) are considered for internal use only, and as such are subject to change  without\nnotice.\n"
                },
                {
                    "name": "Readline Support / History in the Debugger",
                    "content": "As  shipped,  the  only  command-line  history  supplied  is a simplistic one that checks for\nleading exclamation points.  However, if you install  the  Term::ReadKey  and  Term::ReadLine\nmodules from CPAN (such as Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full\nediting  capabilities  much  like  those  GNU  readline(3)  provides.   Look for these in the\nmodules/by-module/Term directory on CPAN.   These  do  not  support  normal  vi  command-line\nediting, however.\n\nA  rudimentary  command-line completion is also available, including lexical variables in the\ncurrent scope if the PadWalker module is installed.\n\nWithout Readline support you may see the symbols \"^[[A\", \"^[[C\", \"^[[B\", \"^[[D\"\",  \"^H\",  ...\nwhen using the arrow keys and/or the backspace key.\n"
                },
                {
                    "name": "Editor Support for Debugging",
                    "content": "If  you  have  the  GNU's version of emacs installed on your system, it can interact with the\nPerl debugger to provide an integrated software development environment  reminiscent  of  its\ninteractions with C debuggers.\n\nRecent  versions  of Emacs come with a start file for making emacs act like a syntax-directed\neditor that understands (some of) Perl's syntax.  See perlfaq3.\n\nUsers of vi should also look into vim and gvim, the mousey and windy version, for coloring of\nPerl keywords.\n\nNote that only perl can truly parse Perl, so all such CASE tools fall somewhat short  of  the\nmark, especially if you don't program your Perl as a C programmer might.\n"
                },
                {
                    "name": "The Perl Profiler",
                    "content": "If  you  wish  to  supply  an alternative debugger for Perl to run, invoke your script with a\ncolon and a package argument given to the -d flag.  Perl's alternative  debuggers  include  a\nPerl  profiler,  Devel::NYTProf,  which  is  available separately as a CPAN distribution.  To\nprofile your Perl program in the file mycode.pl, just type:\n\n$ perl -d:NYTProf mycode.pl\n\nWhen the script terminates the profiler will create a database  of  the  profile  information\nthat you can turn into reports using the profiler's tools. See <perlperf> for details.\n"
                }
            ]
        },
        "Debugging Regular Expressions": {
            "content": "\"use  re  'debug'\"  enables  you  to  see the gory details of how the Perl regular expression\nengine works. In order to understand this typically voluminous output, one must not only have\nsome idea about how regular expression matching works in general, but also  know  how  Perl's\nregular  expressions are internally compiled into an automaton. These matters are explored in\nsome detail in \"Debugging Regular Expressions\" in perldebguts.\n",
            "subsections": []
        },
        "Debugging Memory Usage": {
            "content": "Perl contains internal support for reporting its own memory  usage,  but  this  is  a  fairly\nadvanced  concept  that  requires  some  understanding  of  how memory allocation works.  See\n\"Debugging Perl Memory Usage\" in perldebguts for the details.\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "You do have \"use strict\" and \"use warnings\" enabled, don't you?\n\nperldebtut, perldebguts, perl5db.pl, re, DB, Devel::NYTProf, Dumpvalue, and perlrun.\n\nWhen debugging a script that uses #! and is thus normally  found  in  $PATH,  the  -S  option\ncauses  perl  to  search  $PATH  for  it,  so  you  don't  have  to  type  the path or \"which\n$scriptname\".\n\n$ perl -Sd foo.pl\n",
            "subsections": []
        },
        "BUGS": {
            "content": "You cannot get stack frame information or in  any  fashion  debug  functions  that  were  not\ncompiled by Perl, such as those from C or C++ extensions.\n\nIf  you  alter  your  @ arguments in a subroutine (such as with \"shift\" or \"pop\"), the stack\nbacktrace will not show the original values.\n\nThe debugger does not currently work in conjunction with the -W command-line switch,  because\nit itself is not free of warnings.\n\nIf  you're in a slow syscall (like \"wait\"ing, \"accept\"ing, or \"read\"ing from your keyboard or\na socket) and haven't set up your own $SIG{INT} handler, then you won't  be  able  to  CTRL-C\nyour  way  back  to  the  debugger,  because  the  debugger's  own  $SIG{INT} handler doesn't\nunderstand that it needs to raise an exception to longjmp(3) out of slow syscalls.\n\nperl v5.38.2                                 2026-08-18                                 PERLDEBUG(1)",
            "subsections": []
        }
    },
    "summary": "perldebug - Perl debugging",
    "flags": [],
    "examples": [],
    "see_also": []
}