could you explain a little what the match is doing to parse the lines?

It says: Match a "word", defined as a sequence of one or more letters, hyphens and apostrophes ([...]+). When you find that, return it (()). Repeat (/g). That definition of a word is rather primitve, and may need to be tweaked.

use strict;

my $maxcount;
my $find;
my $file;
my %hash;
my $count;

while (<>) {
  while (/([a-zA-Z'\-]+)/g) {
    my $word = $1;
    $count = ++$hash{lc $word};
    if ($count > $maxcount) {
      $find = $word;
      $maxcount = $count;
    }
  }
}

my $numwords = keys %hash;
print "distinct words: $numwords\n";
print "frequency of most common word: $maxcount\n";
print "common word: $find";

__END__
output of perl script.pl script.pl
==================================
distinct words: 25
frequency of most common word: 7
common word: my
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