my %histogram;
my @wordstofind = qw(
  a am an and are as at be been but by can co de do due each
);

while (<DATA>) {
  my @currentline = split;
  $histogram{$_}++ for @currentline;
}

foreach my $i (@wordstofind) {
  while ( my ($key, $value) = each (%histogram) ) {
    if ($key =~ /\b$i\b/) {
      print "Found $key, $value times\n";
    }
  }
}
__DATA__
hello, I am an elephant. I have a long trunk.

##</code><code>##

Found a, 1 times
Found am, 1 times
Found an, 1 times