That is a very tricky problem to get correct. Natural language is difficult for machines to parse.

If you can live with something that will have trouble with honorifics and abbreviations, but be mostly accurate, you could use a fairly simple regex.

use warnings;
use strict;

my $data;

while (my $line = <DATA>){
    chomp $line;
    $data .= " $line";
    $data  =~ s/  +/ /g;

    if ($data =~ m/(.+?[\?\.\!]('|")?\s)(?=\p{Upper}|\p{Punct})/){
        my $sentence = $1;
        print $sentence,"\n\n";
        substr $data, 0 ,length $sentence, '';
    }
}

print $data if $data;


__DATA__
foomatic99 has asked for the wisdom of the Perl
Monks concerning the following question:
I am looking for a way to chop text at sentence
boundaries. I realize that somebody out there must
have come up with some heuristics for doing this,
though I can't think of any unambiguous terms to
search for something like this. I realize that
nothing in a reasonably light-weight implementation
is going to get it right 100% of the time, but
at least I should be able to find something better
than just cutting at a certain number of bytes.
The text is English, utf8... possibly with HTML
entity references. Can anybody help me out here?
[download]

There are all kinds of cases where this will fail, but it may be good enough.