You can simplify your code quite a bit:

#!/bin/perl -w
use strict;

my %sentence_count;

foreach my $sentence(<DATA>) 
  { $sentence=~ s/^\W+//;                 # remove leading non-words
    my $counter = split(/\W+/,$sentence); # split on non-words sequenc
+e (\W+)
                                          # in scalar context split wi
+ll return 
                                          # the number of elements in 
+the generated list,
                                          # no need to count them ($co
+unt= @word in your 
                                          # example would work too)
    $sentence_count{($counter)}++;
   }
   while ( my($sentence_count,$word_count) = each(%sentence_count)) {
           print ("There are $word_count sentences of $sentence_count 
+words\n"); 
           }
__DATA__
one
one
two, two
two. two
two, two.
three three three
 three three three
three three three.
three three, three
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