Programming Perl, 4th Edition

Lexically Scoped Variables: my

To help you avoid the maintenance headaches of global variables, Perl provides lexically scoped variables, often called lexicals for short. Unlike globals, lexicals guarantee you privacy. Assuming you don’t hand out references to these private variables that would let them be fiddled with indirectly, you can be certain that every possible access to these private variables is restricted to code within one discrete and easily identifiable section of your program. That’s why we picked the keyword my, after all.

A statement sequence may contain declarations of lexically scoped variables. Such declarations tend to be placed at the front of the statement sequence, but this is not a requirement; you may simply decorate the first use of a variable with a my declarator wherever it occurs (as long as it’s in the outermost scope the variable is used). In addition to declaring variable names at compile time, the declarations act like ordinary runtime statements: each of them is executed within the sequence of statements as if it were an ordinary statement without the declarator:

my $name = "fred";
my @stuff = ("car", "house", "club");
my ($vehicle, $home, $tool) = @stuff;

These lexical variables are totally hidden from the world outside their immediately enclosing scope. Unlike the dynamic scoping effects of local (see below), lexicals are hidden from any subroutine called from their scope. This is true even if the same subroutine is called from itself or elsewhere—each ...