You are both doing too much work and too little, assuming I understand what you are trying to achieve.

You don't need to open any files. You may need to recurse into sub-directories. Take a while to study the following code:

#!/usr/bin/perl

use strict;
use warnings;
use File::Find::Rule;

parseDir ('./rootdir');


sub parseDir {
    my ($dir) = @_;
    my @subdirs = File::Find::Rule->directory()->in($dir);

    parseDir ($_) for grep {!/^\.\.?$/ && $_ ne $dir} @subdirs;

    my @files = File::Find::Rule->file()->name('*.*')->in($dir);
    
    return if !@files;
    
    my $listFileName = "";

    open my $listFile, '>', "$dir/List.txt" or return;
    print $listFile join "\n", @files, '';
}
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The key to understanding it is that parseDir calls itself to deal with sub-directories. Having dealt with all sub-directories it then creates the list.txt file for the current directory.