./rootdir
./rootdir/folder1 has files file1.txt, file2.txt,file3.txt..etc 


##</code><code>##

./rootdir/folder2 has  files file1.txt, file2.txt,file3.txt..etc too


##</code><code>##

./rootdir/folder1/List1.txt
 
./rootdir/folder2/List2.txt


##</code><code>##


use strict;
use warnings;

use File::Find::Rule;
my $directory = './rootdir';

my @subdirs = File::Find::Rule->directory->in( $directory );

foreach my $dir (@subdirs) {
        #print "$dir\n";
        next if ($dir eq "..");

       if (-d $dir)
        {

        my $curr_dir = $dir;
        open (my $OUTFILE, '>>', "$curr_dir/List.txt") or die "Cant open '$dir.txt!'  $!";
       
        my @files = File::Find::Rule->file() ->name( '*.*' ) -> in( $dir);

        foreach $_ (@files)
                {
                 print $OUTFILE "$_\n";
                }
        close $OUTFILE;
        }

        }

}