While you should really use a module for this, I will answer the question: Your code is not recursive - it descends only one level, and a correct solution would allow for any number. Here is an example that does what you want. Note that symbolic links to directories are not followed.

#! /usr/bin/perl

use strict;
use warnings;

sub list
{
  my ($dir) = @_;
  return unless -d $dir;

  my @files;
  if (opendir my $dh, $dir)
  {
    # Capture entries first, so we don't descend with an
    # open dir handle.
    my @list;
    my $file;
    while ($file = readdir $dh)
    {
      push @list, $file;
    }
    closedir $dh;

    for $file (@list)
    {
      # Unix file system considerations.
      next if $file eq '.' || $file eq '..';

      # Swap these two lines to follow symbolic links into
      # directories.  Handles circular links by entering an
      # infinite loop.
      push @files, "$dir/$file"        if -f "$dir/$file";
      push @files, list ("$dir/$file") if -d "$dir/$file";
    }
  }

  return @files;
}

print "file=", $_, "\n" for list ("D:");
exit 0;
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