The core of the job is done by the first map which will crypt the 6 words of the string "writing Library. Building consider TO($FOO) *binary*" (split on space) with the numbers at the begining of the list @:: ( obtained with shift ) .

This step can be rewrited as :

@keys=qw'79 11 34 69 41 27';
@list_to_crypt = split/ /, 'writing Library. Building consider TO($FOO
+) *binary*' ;
@result=map{ crypt ($_.' ', shift @keys)  } @list_to_crypt ;
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If you print @result you will see :

79jUStLZ6Ck9c
11anOTHZ33A12
34ERehEMmayDo
69pERLsGUZYU2
41HAcKS9/lrEQ
27eRTYtC6a5HI
[download]

Now, in the same map liverpole pops the value at the end of @:: into $x , which is used into a regexp against the crypted values . This can be rewriten as :

@matcher =  qw'2 4 4 2 5 4' ;
foreach $crypted (@result ) {
$x = pop @matcher ;
$crypted  =~/..(.{$x}).*/g ;
push @final, $1 ;
}
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If you print @final you get :

jUSt
anOTH
ER
pERL
HAcK
eR
[download]

The match is fairly understandable : it could read as "any character", "any character", "remember those next $x any character", "anything till the end of line" . And as it happens, $x selects the characters we need from @result !

And the next map is giving the proper formating .

But there is one last thing that i didn't know here : in the first map, the value returned by the map function is what was matched in the regexp ! Now that's interesting !!