So, here comes a quick trivia question: What shall this little script output? 
my @array = undef;

if(@array) {
 print "defined\n";
} else { 
 print "undefined\n";
}

[download]
To my own surprise, the output was this: 
defined

[download]
I couldn't believe what I saw at first. I just spotted a similar 'bug' in my code where one of my subroutines would return 'undef' in an array assignment statement. My consequent if checks on that array thus failed (at least didn't work as I thought they should).

The trick here is that setting @array to undef doesn't really destroy or 'undefine' that array variable. It instead sets the first element of the array to 'undef'! Therfore, the array _does_ exist or rather it's size is positive (>0), albeit it's first item is 'undef'. At least that's the only way I can manage to explain this wierd behavior.

Setting array to '()' (empty array) on the other hand makes things work: 
my @array = ();

if(@array) {
 print "defined\n";
} else { 
 print "undefined\n";
}

[download]


"There is no system but GNU, and Linux is one of its kernels." -- Confession of Faith

In reply to undef'ing @arrays caveat by vladb

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