use File::Basename  qw( fileparse );
use IO::Dir         qw( );

sub find_unique_name {
   my ($file_name) = @_;

   return $file_name
      if not -e $file_name;

   my ($n, $d, $e) = fileparse($file_name, qr/\.[^.]*/);

   my $max;

   if ($n =~ s/\[(\d+)\]\z//) {
      $max = $1;
   } else {
      $max = 0;
   }

   # XXX Should the program fall back to using
   #     -e in a loop if the dir can't be read?

   my $dh = IO::Dir->new("$d.")
      or die("Unable to list contents of directory \"$d\": $!\n");

   my $re = qr/^\Q$n\E\[(\d+)\]\Q$e\E\z/;
   while (defined(my $f = $dh->read())) {
      $max = $1 if $f =~ /$re/ && $max < $1;
   }

   $max++;

   return "$d${n}[$max]$e";
}