#!/usr/bin/perl

use warnings;
use strict;

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

my $s = $s1 . $s2;
my $m = "";

while(length($s) > 0){
  my $z=substr($s, 0, 1, '');
  if($m !~ m/\Q$z\E/){
    $m .= $z:
  }
}

print "$m\n";