The script is caching values which have been looked up, on the theory that the number of sites looked up is much less than the number of lookups. Memoize is a general solution to apply this to any subroutine.

As for your confusion, sub numtoname, aka numtoname(), is a routine which is the only place the hash %numtoname, aka $numtoname{..}, is referenced. Since they all have different sigils, and can be distinguished, you are allso allowed to have a scalar value, $numtoname, and an array @numtoname, aka $nuumtoname.., but they wouldn't do anything useful, in this case.

The local protects the existing value of $_, so that at the end of the routine, that value is restored. Just within the routine, $_ takes on the value of the first argument. local is an old construct which is only useful for protecting previous values assigned to global variables. If you want an actual local variable, use my.