#!/usr/bin/perl -w

use strict;

sub slurp {
        my  $file = shift;
        open my $fh, '<', $file or return undef;
        local $/ unless wantarray;
        return <$fh>;
}

my @long = slurp 'long';
my @short = slurp 'short';

my %diff;
@diff {@long} = ();
delete @diff {@short};
my @diff = sort keys %diff;

print "@diff\n";