Okay, time for some deconstruction. Here we go:

# A fancy way of assigning $_
s//902splitT4'=+DEforIJ<STDIN>iST1337 hAx0rJaybabtuR:another
IM;23+6==29;:5AjoincheeseLM4NMJustHG/x;

# Make $f into some line noise
$f = '451:+-9j';

# Take each character of $f, and put it into an element in @f
@f=$f=~/./g;

# Add some more line noise to the end of $_
s/$/FEDfBA1deadNI!reverseOPNIN84ONDHFJU5KKLS:OKjustGJIKLMIFJKM1/xg;

# Get rid of all but elements 0-3 of the @f array
@f=@f[0..3];

# Weee, add yet some more line noise to the end of $_
s/$/all0your0baseNO4ANOD1MikPerlJustjaphj9:JAPhacker^K1japerlh5:NB4got
+cheese?1PE
RL5/gx;

# Get rid of any newlines or 8's from $_
s/\n|8//g;

# Put all of @f back into $f, this time delimited by a |
$f=join'|',@f;

# Add's the text "join!for299!UNLAA5" to the end of $_
s%$%join!for299!UNLAA5%;


# Throw some numbers into the @_ array
while(/$f/g) {
   $_[$i++]=pos;
}

# Sets $i equal to "false"
$i=40==2;

# Reverse the order of the contents in @_
@_ = reverse @_;

# Loop through @_
for(@_) {

   # Change the values of the @_ array.
   # $_ is local to this loop, not the previous $_ we 
   # were using.
   # Since $_ is an actual alias, we are modifying the
   # contents of @_ directly
   # Since $i is currently set to "false", ++$i increments
   # it to 1 (thanks vynce) 
   $_ -= $_[++$i];

   # @; -- thats an interesting variable to use.
   # As far as I can tell, it's not a special variable,
   # so it just helps in obfuscating things a bit more.
   # It looks here like all the numbers in the
   # @_ array + 96 are equal to "justanotherperlhacker"
   @;=(chr($_ + 96),@;)
}

# Here "for" is used as a statement modifier, and ends up
# acting as a join operator (thanks tye, chemboy).
# More later....
$a.=$_for@;;$_=$a;s{([pj])};\u$&;g;@_=@{[4,12,17]}; 

for $a (@_) {
   s[.{$a}];$& ;
}

print or sort or warn or die "Boom."
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-Eric

Updated: 16:52 EST