Programming Perl, 4th Edition

Scoped Variable Declarations

Most of the rest of this chapter is about using global variables. Or, rather, it’s about not using global variables. There are various declarations that help you not use global variables—or, at least, not use them foolishly.

We already mentioned the package declaration, which was introduced into Perl long ago to allow globals to be split up into separate packages. This works pretty well for certain kinds of variables. Packages are used by libraries, modules, and classes to store their interface data (and some of their semiprivate data) to avoid conflicting with variables and functions of the same name in your main program or in other modules. If you see someone write $Some::stuff,^[81] he’s using the $stuff scalar variable from the package Some. See Chapter 10.

If this were all there were to the matter, Perl programs would quickly become unwieldy as they got longer. Fortunately, Perl’s three scoping declarators make it easy to create completely private variables (using my or state), or to give selective access to global ones (using our). There is also a pseudodeclarator to provide temporary values to global variables (using local). These declarators are placed in front of the variable in question:

my $nose;
our $House;
state $troopers = 0;
local $TV_channel;

If more than one variable is to be declared, the list must be placed in parentheses:

my ($nose, @eyes, %teeth); our ($House, @Autos, %Kids); state ($name, $rank, $serno); local (*Spouse, $phone{HOME}); ...