#untested, but should work
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;
}
__END__
$ perl 2perl.pl 'ab' '--'
$repl: --
old $str: abcd
new $str: --abcd


##</code><code>##

@_=qw;
Just another Perl hacker,;
;$_=q=print
"@_"= and eval;