$ cat script.pl
use strict;
use warnings;
use Foo;

print "calling Foo::mysub\n";
Foo::mysub();
print "calling Bar::mysub\n";
Bar::mysub();

$ cat Foo.pm
package Foo;
use strict;
use warnings;
use Bar;

sub mysub { print "I'm a sub in ", __PACKAGE__, "\n" }
1;

$ cat Bar.pm
package Bar;
use strict;
use warnings;

sub mysub { print "I'm a sub in ", __PACKAGE__, "\n" }
1;

$ perl script.pl
calling Foo::mysub
I'm a sub in Foo
calling Bar::mysub
I'm a sub in Bar