A matching between the identifiers following package and use is not enforced by Perl, albeit it is a convention which is useful in most cases.

• use Foo; instructs Perl to look in its list of module directories for a file called Foo.pm.
  • Edited to add (thanks tobyink): Then it calls Foo->import.
• package Foo; defines the namespace for the following block (or file, if not in a block).
• I know about the -norequire tag only in context of use parent 'Foo'. In that use case, 'Foo' is both a namespace for inheritance, and also instructs Perl to load Foo.pm behind the scenes. The -norequire suppresses this loading part.

You can easily have many files containing package Foo; and, later in the same file, package Foo::Bar. In your example, you would then say use test_v1; and have all symbols in both packages available. You need to understand, however, that if you also use test_v2; in the same program and if this file also defines stuff in the Foo or Foo::Bar namespaces, then these definitions can overwrite each other (triggering warnings as they do). Here's an example of two different implementations of a Foo::Bar class:

# File test_v1.pm
use strict;
use warnings;

package Foo;

sub new {
   my $class = shift;
   bless { version => 1 }, $class;
}

sub version {
    my $self = shift;
    return $self->{version};
}

package Foo::Bar;

use parent -norequire, 'Foo';

1;
[download]

# File test_v2.pm
use strict;
use warnings;

package Foo;

sub new {
   my $class = shift;
   bless { version => 2 }, $class;
}

sub version {
    my $self = shift;
    return $self->{version};
}

package Foo::Bar;

use parent -norequire, 'Foo';

1;
[download]

...you can then use either of them:

1. perl -Mtest_v1 -E "say Foo::Bar->new->version" # prints 1
2. perl -Mtest_v2 -E "say Foo::Bar->new->version" # prints 2

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