Seekers of Perl Obfuscation

[chromatic]

Not terribly obfuscated, but some people don't realize you can do this yet. The line breaks are important, and good luck getting it to run without being in a file.

#!/usr/bin/perl -w

use strict;

#Just
#Another
#Perl
#Hacker

seek(DATA,0,0);*a=*.;warn map{$a-=chomp,s<.>;q<''>;eieio
;$.<8&&$.>2?"$_ ":''}<DATA>;__END__

Update: mpolo points out there shouldn't be a linebreak after __END__.

[tachyon]

Hmm, truly fascinating stuff. For those that are interested here is an explanation and some code snippets

#!/usr/bin/perl

print <DATA>;
print "\n----\n";
seek(DATA,0,0);
print <DATA>;
__END__
1
2
3

When you run this you see two interesting things:

1
2
3
----
#!/usr/bin/perl

print <DATA>;
print "\n----\n";
seek(DATA,0,0);
print <DATA>;
__END__
1
2
3

First the data after the __END__ tag is read and printed. I used to think you had to use __DATA__ Evidently not. Next the SEEK resets the pointer not to the begining of the special file handle __END__ but to the begining of the file! Personally I would call this a *bug* but one mans bug is another man's feature.

Next consider this code

#!/usr/bin/perl

seek(DATA,0,0);
print map{$. . $_}<DATA>;
__END__

This prints:

5#!/usr/bin/perl
5
5seek(DATA,0,0);
5print map{$. . $_}<DATA>;
5__END__

So within the map $. (the line number special var) is always 5 (the total lines)

Now consider this:

#!/usr/bin/perl -w

seek(DATA,0,0);
*a=*.;
print map{$a-=1;$..$_}<DATA>;
__END__

This prints:

C:\>perl test4.pl
5#!/usr/bin/perl -w
4
3seek(DATA,0,0);
2*a=*.;
1print map{$a-=1; $. . $_}<DATA>;
0__END__

Ahha so the glob assignment *a=*. does several things. Initially in the map $a is set to $. so $a=5. We then decrement $a which changes $.

Finally consider this:

#!/usr/bin/perl -w

use strict;

#Just
#Another
#Perl
#Hacker

seek(DATA,0,0);
*a=*.;
print map{$a-=chomp;s/.//; "$.\t$_\n" }<DATA>;
__END__

This prints:

12      !/usr/bin/perl -w
11
10      se strict;
9
8       Just
7       Another
6       Perl
5       Hacker
4
3       eek(DATA,0,0);
2       a=*.;
1       rint map{$a-=chomp;s/.//; "$.\t$_\n" }<DATA>;
0       _END__

So now we can do a deconstruct:

#!/usr/bin/perl -w

use strict;

#Just              # our data
#Another
#Perl
#Hacker

seek(DATA,0,0);    # seek the begining of the file
*a=*.;             # glob assignment
warn               # same effect as print but includes the line num
map{
$a-=chomp          # chomp return num chars chomped (1) thus $a -= 1
,                  # comma op could be ; without changing result
s                  # substitute
<.>                # first char on line
;q<''>;            # with an expression that evals to null
eieio              # the /ee makes second part eval to null
                   # the /i (ignore case) does not matter.
                   # and the /o compiles once
                   # put them together and you have:
                   # Old Macdonald Had A JAPH E I E I O
;                  # yeah well if you need this explained....
$.<8&&$.>2?"$_ ":''# this is  ($. < 8 and $. > 2) ? "$_ " : "", 
                   # we actually only need lines 5-8 (see above)
}                  # close map
<DATA>;            # map data array (whole file)
__END__



cheers

tachyon