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