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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If you're using a Unix system (e.g. Linux or Solaris) then the easiest way is not entirely Perl, but uses the find command to get the filenames, and Perl for the actual renaming:

find . -type f -exec \
 perl -e 'rename($_, lc) || warn "$_: $!\n" for @ARGV' {} \;
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That turns all the filenames in the current directory and below to lower case. If you're using a system which doesn't have the find command, you can use the find2perl utility (it comes with Perl) to generate a Perl script which will behave the same way. On Windows you can type:

find2perl . -type f -eval "rename $_, lc or warn qq{$_: $!\n}"

and then run the script that it outputs.

One way to do it would be something like this:

#!/usr/bin/perl -w
use strict;

# Open 'find' process to list files recursively with paths
open(FIND, "find |");
while(<FIND>) {
        chomp;
        next if $_ eq $0;        # Don't rename ourself
        rename($_, lc($_));
        }
close(FIND);
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JP

you have to use recursion, because you dont know how much deep all subdirectories goes

#!/usr/bin/perl -w
 
use strict;
 
my $path_to_dir = shift;
dir("$path_to_dir");
 
sub dir {
  opendir(DIR,"$_[0]");
  my @list_of_files = readdir(DIR);
  foreach(@list_of_files) {
   if($_ ne "." && $_ ne "..") {
    if(-d "$_[0]/$_") {
        dir("$_[0]/$_");
    }
    else {
        rename "$_[0]/$_","$_[0]/"."\L$_";
    }
   }
  }
}       
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