Dear Monks,

Here is a simple program employing a simple regex match, which gives me the desired output:

use strict;
use warnings;
use 5.010;

my $str = 'abcd';
my $pattern = '(ab)';

if ($str =~ $pattern) {
    my $repl = "--$1";

    say '$repl: ', $repl;
    say 'old $str: ', $str;

    $str =~ s/$pattern/$repl/;
    say 'new $str: ', $str;
}

--output:--
$repl: --ab
old $str: abcd
new $str: --abcd

[download]

When I get the pattern from the command line, the result is the same:

my $str = 'abcd';
my $pattern = shift;

if ($str =~ $pattern) {
    my $repl = "--$1";

    say '$repl: ', $repl;
    say 'old $str: ', $str;

    $str =~ s/$pattern/$repl/;
    say 'new $str: ', $str;
}

--output:--
$ perl 2perl.pl '(ab)' 
$repl: --ab
old $str: abcd
new $str: --abcd

[download]

But if I also get the replacement from the command line, then there is a problem:

my $str = 'abcd';
my $pattern = shift;

if ($str =~ $pattern) {
    my $repl = shift;

    say '$repl: ', $repl;
    say 'old $str: ', $str;

    $str =~ s/$pattern/$repl/;
    say 'new $str: ', $str;
}

--output:--
$ perl 2perl.pl '(ab)' '--$1'
$repl: --$1
old $str: abcd
new $str: --$1cd

[download]

I've tried using eval, but that doesn't work either:

my $str = 'abcd';
my $pattern = shift;

if ($str =~ $pattern) {
    my $temp = shift;
    my $repl = eval $temp;

    say '$repl: ', $repl;
    say 'old $str: ', $str;

    $str =~ s/$pattern/$repl/;
    say 'new $str: ', $str;
}

--output:--
$ perl 2perl.pl '(ab)' '--$1'
Use of uninitialized value $repl in say at 2perl.pl line 39.
$repl: 
old $str: abcd
Use of uninitialized value $repl in substitution (s///) at 2perl.pl li
+ne 42.
new $str: cd

[download]

How can I get perl to evaluate the replacement string in the context of my program? Note: I also tried the /e and /ee modifiers for s/// with no luck.

In reply to eval problem by 7stud

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