# Foo.pm
package Foo;
use strict;
use warnings;
use Bar;

sub foo { warn 'Foo::foo was called' }

Bar->bar;

1;

##</code><code>##

# Bar.pm
package Bar;
use strict;
use warnings;

sub bar { Foo->foo }

1;

##</code><code>##

$ ls
Foo.pm Bar.pm
$ perl Foo.pm
Foo::foo was called