Some things which often fail to get taken into account:

Words with internal apostrophes. (don't won't, can't shouldn't, you'll, it's, etc.)

Words with non ASCII characters. á, ñ, ÿ, etc.

This does:

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

my $word = qr/(?<!\p{Alnum})\p{Alnum}+(?!\p{Alnum})/;
my %count;

while (<DATA>) {
    my $line = lc $_;
    while ($line =~ /($word('$word)?)/g){
        $count{$1}++;
    }
}

printf "%15s %5d\n", $_, $count{$_} for sort keys %count;

__DATA__
"Hello World!"
"Oh poor Yorick, his world I knew well yes I did"
"Words with internal apostrophes. (don't won't, can't shouldn't, you'l
+l, it's, etc.)"
"Señor Montóya's resüme isn't ápropos."
[download]