This code performs this task for you - It makes use of File::Find to find entries within the given path, ensures that they are actually files (stat and -f _) and then renames them. This could be made more precise by added exclusion matches. eg. return unless (m/\.mp3$/);

#!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 $_, lc($_);
}
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