phpman > perldoc > expand(1)

Found in /usr/share/perl/5.34/pod/perlfaq4.pod
  How do I expand function calls in a string?
    (contributed by brian d foy)

    This is documented in perlref, and although it's not the easiest thing
    to read, it does work. In each of these examples, we call the function
    inside the braces used to dereference a reference. If we have more than
    one return value, we can construct and dereference an anonymous array.
    In this case, we call the function in list context.

        print "The time values are @{ [localtime] }.\n";

    If we want to call the function in scalar context, we have to do a bit
    more work. We can really have any code we like inside the braces, so we
    simply have to end with the scalar reference, although how you do that
    is up to you, and you can use code inside the braces. Note that the use
    of parens creates a list context, so we need "scalar" to force the
    scalar context on the function:

        print "The time is ${\(scalar localtime)}.\n"

        print "The time is ${ my $x = localtime; \$x }.\n";

    If your function already returns a reference, you don't need to create
    the reference yourself.

        sub timestamp { my $t = localtime; \$t }

        print "The time is ${ timestamp() }.\n";

    The "Interpolation" module can also do a lot of magic for you. You can
    specify a variable name, in this case "E", to set up a tied hash that
    does the interpolation for you. It has several other methods to do this
    as well.

        use Interpolation E => 'eval';
        print "The time values are $E{localtime()}.\n";

    In most cases, it is probably easier to simply use string concatenation,
    which also forces scalar context.

        print "The time is " . localtime() . ".\n";

  How do I expand tabs in a string?
    You can do it yourself:

        1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;

    Or you can just use the Text::Tabs module (part of the standard Perl
    distribution).

        use Text::Tabs;
        my @expanded_lines = expand(@lines_with_tabs);

  How can I expand variables in text strings?
    (contributed by brian d foy)

    If you can avoid it, don't, or if you can use a templating system, such
    as Text::Template or Template Toolkit, do that instead. You might even
    be able to get the job done with "sprintf" or "printf":

        my $string = sprintf 'Say hello to %s and %s', $foo, $bar;

    However, for the one-off simple case where I don't want to pull out a
    full templating system, I'll use a string that has two Perl scalar
    variables in it. In this example, I want to expand $foo and $bar to
    their variable's values:

        my $foo = 'Fred';
        my $bar = 'Barney';
        $string = 'Say hello to $foo and $bar';

    One way I can do this involves the substitution operator and a double
    "/e" flag. The first "/e" evaluates $1 on the replacement side and turns
    it into $foo. The second /e starts with $foo and replaces it with its
    value. $foo, then, turns into 'Fred', and that's finally what's left in
    the string:

        $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'

    The "/e" will also silently ignore violations of strict, replacing
    undefined variable names with the empty string. Since I'm using the "/e"
    flag (twice even!), I have all of the same security problems I have with
    "eval" in its string form. If there's something odd in $foo, perhaps
    something like "@{[ system "rm -rf /" ]}", then I could get myself in
    trouble.

    To get around the security problem, I could also pull the values from a
    hash instead of evaluating variable names. Using a single "/e", I can
    check the hash to ensure the value exists, and if it doesn't, I can
    replace the missing value with a marker, in this case "???" to signal
    that I missed something:

        my $string = 'This has $foo and $bar';

        my %Replacements = (
            foo  => 'Fred',
            );

        # $string =~ s/\$(\w+)/$Replacements{$1}/g;
        $string =~ s/\$(\w+)/
            exists $Replacements{$1} ? $Replacements{$1} : '???'
            /eg;

        print $string;