rand takes an optional argument. The argument you give it is [10], which is a reference to an (anonymous) array. Now, rand uses its argument as if it were a number. Perl happily casts types for you, and if use a reference (in this case [10]) as a number, you get its memory address. Which is a pretty big number. If you give rand a big number as argument, you get back a random number from 0 up till that big number.

And that's exactly what you are seeing.