I would use readdir to read the names of the files that need to be renamed.
Then I'd only glob for those files that would potentially collide.

  my $existingfiles_dir;
  my $newfiles_dir;
  my $dest_dir;

  opendir XFD, $existingfiles_dir or die "read $existingfiles_dir : $!
+";
  opendir NFD, $newfiles_dir or die "read $newfiles_dir : $!";
  for my $file ( readdir NFD )
  {
    # parse
    my( $num, $ver, $ext ) = $file =~ /(\d+x)(\d*)(\..*)/;

    # find the last of the files it might collide with:
    my( $last ) = 
      sort { $b <=> $a } # sort version numbers descending
      map { /^$num(.*)\Q$ext/ ? $1 : () }
      readdir XFD;
    rewinddir XFD;

    # take the next available number:
    $ver = $last ? $last + 1 : '';

    rename $file, $num.$ver.$last;
  }
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jdporter
The 6th Rule of Perl Club is -- There is no Rule #6.