See the current Perl documentation for prototype.
Here is our local, out-dated (pre-5.6) version:
prototype - get the prototype (if any) of a subroutine
Returns the prototype of a function as a string (or undef if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve.
FUNCTION is a string starting with
CORE::, the rest is taken as a name for Perl builtin. If builtin is not overridable (such as
qw//) or its arguments cannot be expressed by a prototype (such as
system()) - in other words, the builtin does not behave like a Perl function -
returns undef. Otherwise, the string describing the equivalent prototype is returned.