Any variable you see that has a double colon (Foo::bar) or apostrophe (Foo'bar) in it is a package variable.    Any variable created with my is not a package variable.

$ perl -le'
use warnings;
use strict;

package Foo; # this creates a package called Foo
my $Foo::bar = "Hello World";
'
"my" variable $Foo::bar can't be in a package at -e line 6, near "my $
+Foo::bar "
Execution of -e aborted due to compilation errors.
[download]

$ perl -le'
use warnings;
use strict;

package Foo; # this creates a package called Foo
$Foo::bar = "Hello World";

package main; # goes back to default package of class
print $Foo::bar; # this prints $bar form package Foo
'
Hello World
[download]