This will uc the beginning of sentences and take care of the pronoun 'I'. Sorry it won't handle proper nouns :).

#!/usr/bin/perl

use warnings;
use strict;

my $string = "HEY I WANT TO YELL EVERYTHING. MY MOTHER DID NOT pay ME 
+ENOUGH ATTENETION";

$string = join '. ',map {ucfirst} (split(/\. /,lc($string)));
$string =~ s/\bi\b/I/g;
$string .= '.';

print $string;
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UPDATE: Ahh... you can also repeat this to cover different ending punctuation.

$string = join '. ',map {ucfirst} (split(/\. /,lc($string)));
$string = join '? ',map {ucfirst} (split(/\? /,lc($string)));
$string = join '! ',map {ucfirst} (split(/\! /,lc($string)));
$string =~ s/\bi\b/I/g;
$string .= '.';
[download]

grep

Unix - where you can thrown the manual on the keyboard and get a command