my @a = ( 1,1,1,1,1,2,2,2,3,3,4,5,6);
my @b = ( 1,2,3,4,5,6 );

my %d;
my @difference;

$d{ $_ } ++ for @a;
$d{ $_ } -- for @b;

for my $key ( keys %d )
{
    next if ( $d{ $key } <= 0 );
    push @difference, $key for ( 1 .. $d{ $key } );
}

print sort @difference;