use strict;
use warnings;
use Data::Dumper;

my @stuff1 = qw/a b c d/;
my @stuff2 = qw/a d/;

my %seen;          
my @stuff1_only; 
my $element;

foreach $element(@stuff2) {
          $seen{$element} = 1;
}

foreach $element (@stuff1) {
          unless ($seen{$element}) {
                  push (@stuff1_only, $element);
          }
}

print Dumper \@stuff1_only;