this is not realy true. you get the number of elements of an array by setting the array in scalar context. $#array gives you the the position of the last array element, which is mostly not the same.
to get the size of a nested array, you have to understand references and how perl handles lists of lists. for the lazy, here is an example.

print scalar @{$foo{key1}->[0]}, "\n";

for those who want to know whats going on, start by reading perlreftut, perldsc and perllol.