my $filename = "xyz";
for (1..10) {
  print qq[Filename is "$filename"\n];
  $filename = &newname($filename,1);
}

##</code><code>##

sub newname() {

  my $filename  = shift || "aaaa";
  my $sign      = shift || 1;
  my $overwrite = shift;
  {
    my $x = 1;
    1 while substr($filename,-$x++,1) eq ($sign<1 ? "a":"z");
    if (--$x > length $filename) { $filename = tmpnam(); }
    else {
      substr($filename,-$x--,1)=chr(ord(substr($filename,-$x,1))+$sign);
      substr($filename,-$x--,1)=($sign<1 ? "z":"a") while $x;
    }
    redo if !$overwrite and -e $filename;
    return $filename;
  }
}