Perl in a Nutshell, 2nd Edition

Prototypes

Prototypes allow you to design your subroutines to take arguments with constraints on the number of parameters and types of data. To declare a function with prototypes, use the prototype symbols in the declaration line like this:

sub addem ($$) {
...
}

In this case, the function expects two scalar arguments. The following table gives the various prototype symbols :

Symbol	Meaning
$	Scalar
@	List
%	Hash
&	Anonymous subroutine
*	Typeglob

A backslash placed before one of these symbols forces the argument to be that exact variable type. For instance, a function that requires a hash variable would be declared like this:

sub hashfunc (\%);

Unbackslashed @ or % symbols act exactly alike and will eat up all remaining arguments, forcing list context. Likewise, a $ forces scalar context on an argument, so taking an array or hash variable for that parameter would probably yield unwanted results.

A semicolon separates mandatory arguments from optional arguments. For example:

sub newsplit (\@$;$);

requires two arguments: an array variable and a scalar. The third scalar is optional. Placing a semicolon before @ and % is not necessary since lists can be null.

A typeglob prototype symbol (*) will always turn its argument into a reference to a symbol table entry. It is most often used for filehandles.