Ah, there's nothing like a bit of recursion to add some obfuscation. A really, really clever obfu. First you need to understand what/how this code prints:

$_ = "_";
print;
tr/_/J/;
print .'';
tr/J/u/;
print .''.tr/u/s/;
print .tr/s/t/;
print .tr/t/ /;
[download]

You can see this prints _Just by virtue of the fact that print uses $_ if no value is specified. As there is no value to the left of the concatenation that is what is used. Here is the decon:

# declare some protos so we can call the subs J A P and H
# without an & prefix or () suffix -> ie as barewords.
sub J;sub A;sub P;sub H; 
# {s __\__ ==  J } is the same as:
s//_/;
J;
# the s//_/ substitutes '_' into $_
# and the == J checks for equality to
# the return value of the subroutine J
# called as a bareword by virute of the 
# prototyping. The equiality is false BTW :-)
# effectively we just call sub J
sub J {
    print   # print
    .       # concatenate
    y:s:S:  # tr/s/S/ important because label 's' does not exist
    .       # concatenate
    goto$_; # go to '_:' label which follows
    _:      # label
    y:_:J:  # tr/_/J/ change value in $_ to 'J'
    .       # concatenate
    J;      # recursively call sub J but now $_ = 'J'
            # so goto $_ goes to J: below
    J:      # more of the same.....
    y:J:u:  # change label marker in $_ to 'u'
    .
    J;      # recurse
    u:      # label
    y:u:s:  # change label 'u' to 's' which does not exist
            # but first tr before goto changes $_ 's' to 'S' 
            # which does exist below so more of the same!
    .
    J;      # recurse
    S:      # label
    y:S:t:  # change label marker in $_ to 't'
    .
    J;      # recurse
    t:      # label
    y:t: :  # change label marker in $_ to ' ' 
    ==  A   # call the next sub A via a bareword
}
  sub A {  print.
      y: :_:.goto$ # change $_ from ' ' as left in the last sub
                   # to '_' allowing us to find the label '_:'
      _;_:y:_:a:.  # same theme repeated
      A;a:y:a:n:.
      A;n:y:n:o:.
      A;o:y:o:t:.
      A;t:y:t:h:.
      A;h:y:h:e:.
      A;e:y:e:r:.
      A;r:y:r: :   

  ==  P }
  sub P {  print.
      y: :_:.goto$ # as above
      _;_:y:_:P:.
      P;P:y:P:e:.
      P;e:y:e:r:.
      P;r:y:r:l:.
      P;l:y:l: :   

  ==  H }
  sub H {  print.
      y: :_:.goto$ # as above
      _;_:y:_:h:.
      H;h:y:h:a:. 
      H;a:y:a:c:.
      H;c:y:c:k:.
      H;k:y:k:e:.
      H;e:y:e:r:.
      H;r:y:r:_:. # final transformation changes $_ to '_'
      print;exit} # so we print symetrical '_' and exit
[download]

Recursion and goto what a choice combo to make spagetti

cheers

tachyon

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