# One.pm
package One;

sub new {
   my $class = shift;
   print "creating object of class $class\n";
   bless {}, $class;
}

1;

# Two.pm
package Two;

@ISA = 'One';

1;

# interface.pl
use One;
use Two;

my $one = One->new();
my $two = Two->new();