The implementation of what you need is perhaps a bit easier than it might appear from the FAQ that choroba rightly points out. The second "foreach" is just a grep. Consider:

my %count;
$count{$_}++ foreach (@ServicesOnMachine, @ServiceCheckList);
my @intersection = grep{$count{$_} >=2} keys %count;
print "@intersection\n";
#prints: sendmail httpd ypbind
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