Something which none of the other solutions posted here so far takes into account, is words with internal apostrophes. (don't won't, can't shouldn't, you'll, it's, etc.) And most don't deal with non-ascii characters.

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"
"don't won't, can't shouldn't, you'll, it's, etc."
"Señor Montóya's resüme isn't ápropos."
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