Careful Evaluation

[iamcal]

A bit of a silly one. I dare somebody to deconstruct it :)

$$=q{$_=unpack'u',q|L)%\]<2$D7STG<')I;G0@(DIU<W0@86YO=&AE<B!
097)L(&AA8VME<B(G(3L`|;};$}=q{$_=unpack'u',q|<)%\])R1?/7$A*"
1?*3TH)"0]?B\N*B]G*2$G.P``|;};$$=~s;\n;;g;$;=q;$_=$};;$_='$_
=$;;';$}=~s;\n;;g;

eval;eval;eval;eval;eval;eval;eval;eval;eval;eval;

[VSarkiss]

All right, nobody else has, so I'll take the bait. ;-)

For those who may not have seen them before (*cough* young whipper-snappers), uuencode and uudecode are encoding and decoding mechanisms that were originally developed to allow transmitting binary files via uucp over lines that were not always eight-bit-clean. Just like MIME.

Now, on to the real decode. I've separated the lines to make them a little easier to read.

#
# Set $$ -- usually the PID of the running Perl, but not a read-only
# variable -- to a strange string: first part looks like code,
# but latter half looks like junk.  If you uudecode the junk (minus
# the embedded newline) you get:
#
#   $_=q!$_='print "Just another Perl hacker"'!;
#
$$ = q{$_=unpack'u',q|L)%\]<2$D7STG<')I;G0@(DIU<W0@86YO=&AE<B!
097)L(&AA8VME<B(G(3L`|;};

#
# Set $} -- another strange variable -- to another strange string.
# This time the uuencoded part (again minus \n) is
#
#   $_='$_=q!($_)=($$=~/.*/g)!';
#
$} = q{$_=unpack'u',q|<)%\])R1?/7$A*"
1?*3TH)"0]?B\N*B]G*2$G.P``|;};

#
# The next few lines remove embedded newlines in $$ and $} (first
# and fourth lines), and set these variables to these values:
# $;    $_=$};        (second line)
# $_    $_\n=$;;      (third line, including embedded newline)
#
$$=~s;\n;;g;
$;=q;$_=$};;
$_='$_
=$;;';
$}=~s;\n;;g;

#
# Each of these evals executes the code in $_.  Before the first,
# $_ contains "$_\n=$;;" as shown above.  Here are the values of $_
# after each eval completes.
#
eval;
#     $_=$};
eval;
#     $_=unpack'u',q|<)%\\])R1?/7$A*"1?*3TH)"0]?B\\N*B]G*2$G.P``|;
eval;
#     $_='$_=q!($_)=($$=~/.*/g)!';
eval;
#     ($_)=($$=~/.*/g)
# BTW, the parens and "=~ /.*/g" do nothing.
# Executing this is the same as "$_ = $$".
eval;
#     $_=unpack'u',q|L)%\\]<2$D7STG<\')I;G0@(DIU<W0@86YO=&AE<B!097)L(&
+AA8VME<B(G(3L`|;
eval;
#     $_=q!$_='print "Just another Perl hacker"'!;
eval;
#     $_='print "Just another Perl hacker"'
eval;
#     print "Just another Perl hacker"
eval;
#     The final eval just executes the contents of $_ seen above.
eval;

BTW, if you try to execute this in the debugger, beware: setting $$ causes it to fork. Fiendishly clever!

[Brovnik]

I dare somebody to deconstruct it :)

OK. First, tidy up syntax and delimiters and rename some variables.

$$ = q{$_=unpack'u',q|L)%\]<2$D7STG<')I;G0@(DIU<W0@86YO=&AE<B!097)L(&A
+A8VME<B(G(3L`|;};
$b = q{$_=unpack'u',q|<)%\])R1?/7$A*"1?*3TH)"0]?B\N*B]G*2$G.P``|;};
#$$ =~ s/\n//g;
$a = '$_=$b';
$_ = '$_=$a';
#$} =~ s/\n//g;

eval; eval; eval; eval; eval; eval; eval; eval; eval; eval;

The commented lines aren't needed if the q|| are on one line.

So, (this is easier to read starting from the bottom of the tidied code) the first 3 evals set $_ to $a and then $b then eval the unpack inside $b.

The unpack produces $_='$_=q!($_)=($$=~/.*/g)!';, which, after 3 more iterations sets $_ to $$ (the unpack on the first line).
This unpack sets $_=q!$_='print "Just another Perl hacker"'!;, which, after another 2 iterations gives us the familiar
print "Just another Perl hacker" which duly evals.

Hope that is sufficient explanation.
--
Brovnik