use strict;
use warnings;

my @p = ( 1,1,1,1,1,2,2,2,3,3,4,5,6);
my @q = ( 1,2,3,4,5,6 );

my ($px, $qx) = (0, 0);

my @diff;
while (1) {
    if ($qx >= @q){
        push @diff, @p[$px .. @p-1];
        last;
    } elsif ( $p[$px] == $q[$qx] ) {
        $px++;
        $qx++;
    } else {
        push @diff, $p[$px++];
    }
}

print "p: @p\n";
print "q: @q\n";
print "d: @diff\n";

##</code><code>##

while ($qx < @q) {
    if ( $p[$px] == $q[$qx] ) {
        $px++;
        $qx++;
    } else {
        push @diff, $p[$px++];
    }
}
push @diff, @p[$px .. @p-1];