You need a hash:

my @old_stock1 = ('products','here');
my @old_stock2 = ('products','here');

# define hash to store quantities
my %new_stock;

# iterate through old
for (@old_stock1, @old_stock2) {

  # add one to count for this element
  # (creating it if it doesn't exist)
  $new_stock{$_}++;

}
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Then do what you need to do with the hash, eg:

for (keys %new_stock) {
  print "$_ = $new_stock{$_}\n";
}
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would list name and quantity of each item.

cLive ;-)

ps - Update - oops, just re-read the question. See below (but the principal still stands :) - pjf seems to have covered it.