Let's start by eliminating unnecessary steps and by not storing more values into memory than necessary. By getting rid of the fluff, you might be able to see the problem and solution more clearly. 

#!/usr/bin/perl
use strict;
use warnings;

my %histogram = map { $_ => 0 } qw(
    a am an and are as at be been but by can co de do due each
); # hash of the words to find so we can do an O(1) lookup for them

while ( <> ) {
    chomp;
    for ( split ) {
    # split returns a list we can use directly
        $histogram{ $_ }++ if exists $histogram{ $_ };
        # only store counts for words that matter
    }
}

foreach my $word ( keys %histogram ) {
# keys() will list the keys, and we've already taken care
# of making sure we don't have extra words stored.
# Now there's no need to do two loops and check an array
# against a hash.
    print "Found $word, $histogram{$word} times.\n";
}

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In reply to Re: 'Simple' comparing a hash with an array by mr_mischief
in thread 'Simple' comparing a hash with an array by Anonymous Monk

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