#!/usr/bin/env perl

use strict;
use warnings;

my $input = "I am a GIRL";
my $output = [];

# split input by non-words ... if you need to keep non-word 
# characters then you'll need a different regex
foreach my $word ( split( /\W+/, $input ) ) {
  # if the word is all upper case and it's not a single letter
  if( $word =~ /^\p{Uppercase}+$/ && length( $word ) > 1 ) {
    push( @$output, $word );
  } else {
    push( @$output, lc $word );
  }
}

print join( ' ', @$output ), "\n";