What happens on this my declaration?

Question

I know the title sounds funny, but I found this snippet somewhere:

my MyPackage $p1 = MyPackage->new;

What role does the name of the package serve in front of $p1?

EDIT: I'm running perl 5.10.1.

Answer 1

It checks for a package with the same name, and, as of now, is tied to the fields pragma which helps check for typos in field names.

For example:

package MyPackage;
use fields qw/ foo bar /;
sub new { fields::new(shift) }

Then if you try to run

use MyPackage;
my MyPackage $p1 = MyPackage->new;
print $p1->{notpresent}, "\n";

you get

No such class field "notpresent" in variable $p1 of type MyPackage at ...

Answer 2

From http://perldoc.perl.org/functions/my.html:

my TYPE EXPR : ATTRS

A my declares the listed variables to be local (lexically) to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses.

The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of fields pragma, and attributes are handled using the attributes pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module.