Your match is finding portions of a word: $key =~ /$i/ will match when $key is "elephant" and $i is "a" because "elephant" contains an "a". To match on whole words only, use /b (word boundary - see perlre) before and after your search term: 
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.

[download]

Output: 

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

[download]

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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.