Slightly OT, but probably worth mentioning. Eval is indeed cool, but it has its evil side. E.g the following code:

#code a
my $function = $node->{'activationfunction'};
my $functionCall ="\$value = \$network->$function(\$value);";
eval($functionCall);

#code b

my $function = $node->{'activationfunction'};
$value = $network->$function($value);
[download]

Eval saved me once upon a time from having to learn that code b was possible. BUT, since I was calling this tens of thousands of times in normal program execution, and hence invoking the compiler every time, once I did learn that code b was possible, the program speeded up by several hundred fold!

The moral - eval is cool, but very very slow!