I'm finding when the hash gets to be more than 27,000, I get the out-of-memory error.

hum, that's really not that big. Especially if it just hold URLs. You should see the following surpass that number in no time.

my $i;
my %urls;
for (;;)
   ++$url{"http://www.perlmonks.org/?node_id=".++$i};
   print("$i\n") if $i % 1000 == 0;
}
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Tie a shoelace around a shoebox and wave a magic wand over it.

Actually, that's quite an apt description!

tie is a form of magic (yes, that's really what it's called) that ties methods to a variable (shoebox) such that those functions are called in response to the different actions taken on that variable. For example, $tied = $x; calls the tied variable's STORE method.

It's easier to understand if you look at it from the other end. tie allows a module to use a Perl variable as its interface. Instead of $obj->store($x), it allows $obj = $x;.