#!/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";
}