$_ = $";
  # copy $", which is a space, to $_, for convenience later

$^ = A;
  # assign 'A', the first label, to $^

&{':'};
  # use a symbolic reference to call the sub named :
  # this starts the process... (see below)

sub AUTOLOAD {
  # the AUTOLOAD sub, called when an undefined subroutine is called

  print $AUTOLOAD =~ /:+(.?)/s;
    # $AUTOLOAD holds the name of the undefined subroutine, including 
+package
    #   i.e. &J calls AUTOLOAD and sets $AUTOLOAD to 'main::J'
    # find the colons, match the next character, and print it
    #   (when $AUTOLOAD ends in a colon, prints a null string)

  goto $^++;
    # goto the label specified by $^
    # post-increment $^ for next time
}

# the rest of the script consists of labels and statements,
#   in some non-alphabetical order - that's okay, because
#   the goto calls are in alphabetical order
# to make the code easier to follow I'll reorder it starting here

&{':'};
  # start the process by calling the sub :
  # a null string is printed, and goto jumps to label A

A: &J;
  # label A: call the sub J
  # 'J' is printed, and goto jumps to label B

B: &u;
  # label B: call the sub u
  # 'u' is printed, and goto jumps to label C

# this continues for some time...

E: &$_;
  # label E: call the sub [space] with a symbolic reference
  # ' ' is printed, and goto jumps to label F

  # etc...

Z: &{$/};
  # label Z: call the sub [newline] with a symbolic reference
  # "\n" is printed (thanks to the /s on the regex),
  #   and goto jumps to label AA

AA: rjk;
  # rjk is a bareword, not a subroutine call,
  # so the script ends
[download]