#!/usr/bin/perl

use warnings;
use strict;

my $s1 = 'abcdef';
my $s2 = 'efghijk';

my $s = $s1 . $s2;

my $a;
my $i;

my $m = "";


for ($i = 0; $i < length($s); $i++) {
    $a = substr($s, $i, 1);
    if ($m !~ m/\Q$a\E/) {
        $m .= $a;
    }
}

print "$m\n";