Programming Perl, 4th Edition

Prototypes

Perl lets you define your own functions to be called like Perl’s built-in functions. Consider push(@array, $item), which must tacitly receive a reference to @array, not just the list values held in @array, so that the array can be modified. Prototypes let you declare subroutines to take arguments just like many of the built-ins―that is, with certain constraints on the number and types of arguments. We call them “prototypes”, but they work more like automatic templates for the calling context than like what C or Java programmers would think of as prototypes. With these templates, Perl will automatically add implicit backslashes, or calls to scalar, or whatever else it takes to get things to show up in a way that matches the template. For instance, if you declare:

sub mypush (+@);

then mypush takes arguments exactly like push does. For this to work, the declaration of the function to be called must be visible at compile time. The prototype only affects the interpretation of function calls when the & character is omitted. In other words, if you call it like a built-in function, it behaves like a built-in function. If you call it like an old-fashioned subroutine, then it behaves like an old-fashioned subroutine. The & suppresses prototype checks and associated contextual effects.

Because prototypes are taken into consideration only at compile time, it naturally falls out that they have no influence on subroutine references like \&foo or on indirect subroutine calls like &{$subref} ...