perldoc > access(2)

🚀 Quick Reference

Use Case	Command	Description
Get first character of a string	substr($string, 0, 1)	Access the first character of a string.
Replace part of a string	substr($string, 13, 4, "replacement")	Change a substring using the fourth argument.
Access global variable when lexical is in scope	$main::var	Explicitly reference the package variable.
Bring dynamic variable into lexical scope	our $var;	Use our() to alias a global variable.
Catch undefined variable accesses	use warnings FATAL => qw(uninitialized);	Promote warnings to errors for uninitialized variables.

📂 Found in /usr/share/perl/5.34/pod/perlfaq4.pod

✂️ How can I access or change N characters of a string?

You can access the first characters of a string with substr(). To get the first character, for example, start at position 0 and grab the string of length 1.

my $string = "Just another Perl Hacker";
my $first_char = substr( $string, 0, 1 );  #  'J'

To change part of a string, you can use the optional fourth argument which is the replacement string.

substr( $string, 13, 4, "Perl 5.8.0" );

You can also use substr() as an lvalue.

substr( $string, 13, 4 ) =  "Perl 5.8.0";

📂 Found in /usr/share/perl/5.34/pod/perlfaq7.pod

🔍 How can I access a dynamic variable while a similarly named lexical is in scope?

If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is not the dynamic $var in the current package, but rather the one in the "main" package, as though you had written $main::var.

use vars '$var';
local $var = "global";
my    $var = "lexical";

print "lexical is $var\n";
print "global  is $main::var\n";

Alternatively you can use the compiler directive our() to bring a dynamic variable into the current lexical scope.

require 5.006; # our() did not exist before 5.6
use vars '$var';

local $var = "global";
my $var    = "lexical";

print "lexical is $var\n";

{
    our $var;
    print "global  is $var\n";
}

🚨 How can I catch accesses to undefined variables, functions, or methods?

The AUTOLOAD method, discussed in "Autoloading" in perlsub lets you capture calls to undefined functions and methods.

When it comes to undefined variables that would trigger a warning under "use warnings", you can promote the warning to an error.

use warnings FATAL => qw(uninitialized);