#!d:/perl/bin/perl -w

use File::Find;
use strict;

find ({
    'wanted'    =>  \&renamefile,
}, '/path/to/sub/directory');

sub renamefile {
    my ($dev, $ino, $mode, $nlink, $uid, $gid);
    return unless (($dev, $ino, $mode, $nlink, $uid, $gid) = lstat($_));
    return unless (-f _);
    rename $_, lc($_);
}