#!/usr/bin/perl -w

  use strict;

  my ($file, %rules, $string, $rewrite, $angle, $i);

  $file = "gosper";
  
  # Peano-Gosper curve.
  $rules{"X"} = "X+YF++YF-FX--FXFX-YF+";
  $rules{"Y"} = "-FX+YFYF++YF+FX--FX-Y";
  $string = "FX"; $angle = 60; $i = 4;

  foreach (1..$i) {
    $rewrite = "";
    for (split //,$string) {
      $rewrite .= $rules{$_} || $_;
    }
    $string = $rewrite;
  }

  open PS,">$file.ps";
  print PS "%#\n\n";
  print PS "306 396 moveto\n";
  for my $ch (split //,$string) {
    if ($ch eq 'f') {
      print PS "0 1 rlineto\n";
    } elsif ($ch eq 'F') {
      print PS "0 1 rmoveto\n";
    } elsif ($ch eq '+') {
      print PS "360 $angle rotate\n";
    } elsif ($ch eq '-') {
      print PS "360 $angle neg rotate\n";
    } elsif ($ch eq '!') {
      print PS "-1 1 scale\n";
    } elsif ($ch eq '[') {
      print PS "gsave\n";
    } elsif ($ch eq ']') {
      print PS "stroke\ngrestore\n";
    }
  }
  print PS "stroke\nshowpage";
  close PS;
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Note that you need to replace uppercase F with lowercase f to turn on the 'pen'.