... you know, when your mind is like, in a bottle? Anyway, I was trying to do some permutations, and I wanted a recursive solution. I found a snippet of code that does it. However, I don't like putting code into my program without understanding what it does. This code gives me memories of my lisp class in college: 

#!/usr/bin/perl

print "permute:\n";
print "[", join(", ", @$_), "]\n" for permute([1,2,3], [4,5,6], [7,8,9
+]);

sub permute 
{
    my $last = pop @_;

    unless(@_) 
    {
           return map([$_], @$last);
    }

    return map { 
                 my $left = $_; 
                 map([@$left, $_], @$last)
               } 
               permute(@_);
}

[download]
Here's what I can figure out so far. The function goes into recursion until @_ is empty, at which point it returns [[1],[2],[3]] (or is it ([1],[2],[3])??) to the previous level of recursion. At that level, I know that $last is a reference to an array that contains [4,5,6]. From there, it gets sort of confusing (inside the map). This is mainly due to my lack of experience with map. I know what it does in principle, but applying it recursively and with $_, is giving me head-aches! Any help is appreciated. Thanks!

In reply to This recursion is so "mind-bottling" by vivin

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