Depending on your definition of "word" and whether or not you have multiple spaces between "words" something like this might work:

#!/usr/bin/perl -wT
use strict;

my $string = 'abc def GHI JkLMno pQrSTUv';

my @words = map {ucfirst(lc($_))} split(' ',$string);  # Abc Def, etc.
+..
$words[-1] = uc($words[-1]);                           # PQRSTUV
my $newstring = join(' ',@words);                      # back together

print "$newstring\n";

__END__

=head1 OUTPUT
Abc Def Ghi Jklmno PQRSTUV
[download]

You should probably read about uc, lc, and ucfirst.

-Blake