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]

In reply to Re: Converting a text into lowercase but not the word with all CAPS letters. by tybalt89
in thread Converting a text into lowercase but not the word with all CAPS letters. by pdahal

Title:
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