4.7.1 Calling Subroutines

The ampersand (&) is the identifier used to call subroutines. Most of the time, however, subroutines can be used in an expression just like built-in functions. To call subroutines directly:

name(args);                  # & is optional with parentheses
name args;                   # Parens optional if predeclared/imported
&name;                       # Passes current @_ to subroutine

&$subref(args);                 # & is not optional on indirect call
&$subref;                       # Passes current @_ to subroutine

4.7.2 Passing Arguments

All arguments to a subroutine are passed as a single, flat list of scalars, and return values are returned the same way. Any arrays or hashes passed in these lists will have their values interpolated into the flattened list.

Any arguments passed to a subroutine come in as the array @_.

You may use the explicit return statement to return a value and leave the subroutine at any point.

4.7.3 Passing References

If you want to pass more than one array or hash into or out of a function and have them maintain their integrity, then you will want to pass references as arguments. The simplest way to do this is to take your named variables and put a backslash in front of them in the argument list:

@returnlist = ref_conversion(\@temps1, \@temps2, \@temps3);

Returning references is a simple matter of returning scalars that are references. This way you can return distinct hashes and arrays.

4.7.4 Private and Local Variables

Any variables you use in the function that aren't declared private are global variables. In subroutines, you'll often want to use variables that won't be used anywhere else in your program, and you don't want them taking up memory when the subroutine is not being executed. You also might not want to alter variables in subroutines that might have the same name as global variables.

The my function declares variables that are lexically scoped within the subroutine. Lexically scoped variables are private variables that only exist within the block or subroutine in which they are declared. Outside of their scope, they are invisible and can't be altered in any way.

To scope multiple variables at once, use a list in parentheses. You can also assign a variable in a my statement:

my @list = (44, 55, 66);
my $cd = "orb";

4.7.5 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 ($$) {
...
}

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 (\%);

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

sub newsplit (\@$;$);

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