$ cat 938978.pl 
#!/usr/bin/perl
use strict;
use warnings;

while (<DATA>) {
    chomp;
    my ($x, $y) = split /\s+/,$_;
    last if ! defined $y;
    print robo_1($x,$y), "  <$x> <$y>\n";
}

sub robo_1 {
    my %h;
    $h{$_}++ for split//,shift;
    $h{$_}-- for split//,shift;
    my $t=0;
    $t += ($_>0 ? $_ : -$_) for values %h;
    return $t;
}
__DATA__
123456 abcdefghijkl
ARF_ARF_ARF BARKBARKBARK
doom gloom
heartbreak anguish

$ perl 938978.pl
18  <123456> <abcdefghijkl>
11  <ARF_ARF_ARF> <BARKBARKBARK>
3  <doom> <gloom>
13  <heartbreak> <anguish>