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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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 line 42.
new $str: cd