Kid.pm
------
package Kid;
@ISA = (Son);

use Son;

sub new
{
}

Son.pm
-------
package Son;

sub new
{
}

sub getSonSex
{
print("I'm a boy \n");
}

test.pl
-------
use Kid;
use Son;

$kid = Kid->new();

# It's OK
$kid->getSonSex();