Programming Perl, 4th Edition

sub

Named declarations:

sub NAME PROTO ATTRS
sub NAME ATTRS
sub NAME PROTO
sub NAME

Named definitions:

sub NAME PROTO ATTRS BLOCK
sub NAME ATTRS BLOCK
sub NAME PROTO BLOCK
sub NAME BLOCK

Unnamed definitions:

sub PROTO ATTRS BLOCK
sub ATTRS BLOCK
sub PROTO BLOCK
sub BLOCK

The syntax of subroutine declarations and definitions looks complicated, but it is actually pretty simple in practice. Everything is based on the syntax:

sub NAME PROTO ATTRS BLOCK

All four fields are optional; the only restrictions are that the fields that do occur must occur in that order, and that you must use at least one of NAME or BLOCK. For the moment, we’ll ignore the PROTO and ATTRS; they’re just modifiers on the basic syntax. The NAME and the BLOCK are the important parts to get straight:

• If you have just a NAME and no BLOCK, it’s a predeclaration of that name (but if you ever want to call the subroutine, you’ll have to supply a definition with both a NAME and a BLOCK later). Named declarations are useful because the parser treats a name specially if it knows it’s a user-defined subroutine. You can call such a subroutine either as a function or as an operator, just like built-in functions. These are sometimes called forward declarations.

• If you have both a NAME and a BLOCK, it’s a standard named subroutine definition (and a declaration, too, if you didn’t declare the name previously). Named definitions are useful because the BLOCK associates an actual meaning (the body of the subroutine) with the declaration. That’s ...