#! /usr/bin/perl
use warnings;
use strict;

my $input = 'a';

my $doesnt_work = $input;

for (1 .. 10) {
    my $works = $input;

    $works       =~ s/a/$_/;
    $doesnt_work =~ s/a/$_/;

    print "Works: $works\n", "Doesn't work: $doesnt_work\n";
}