----------
### File A.pm
package A;
sub import { warn "A imported"; }
----------
### File X.pm
package X;
use base qw(A);
sub import { warn "X imported"; }
----------
### File Y.pm
package Y;
use A;
use base qw(A);
sub import { warn "Y imported"; }
-----------
### File Z.pm
package Z;
use A;
@ISA = qw(A);
sub import { warn "Z imported"; }
----------