Taking a guess about your contradictory requirements:

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1194583

use strict;
use warnings;

while(<DATA>)
  {
  s/ \b (?![A-Z]{2,}\b) (\w+) \b /\L$1/gx;
  print;
  }

__DATA__
I am a GIRL
A test WITH Multiple conDITions AND other tests.
[download]