It associates a "type" with the variable — e.g. for use with fields: 

package Foo;
use fields qw(foo bar);

package main;

my Foo $foo = {};

$foo->{baz} = 42;

[download]
$ ./882116.pl
No such class field "baz" in variable $foo of type Foo at ./882116.pl 
+line 8.

[download]

In reply to Re: Specifying the class of an object in the declaration by Anonyrnous Monk
in thread Specifying the class of an object in the declaration by FalseVinylShrub

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