sub str_replace {
   my ($string, $pattern, $replace) = @_;
   
   $pattern ||= "";
   $replace ||= "";

   my $patternlen = length $pattern;
   my $replacelen = length $replace;
 
   my $pos = 0;
   while(($pos = index($string, $pattern, $pos)) > -1) {
      $string = substr($string, 0, $pos) . $replace . substr($string, $pos + $patternlen);
      $pos += $replacelen;
   }

   return $string;
}

str_replace("abcdefghijk", "cde", "");