package Base;
sub init {
    my $class = shift;
    my $is_func = $class.'::is_'.lc($class);
    no strict 'refs';
    *$is_func = sub { return 1 };
}
...
package Foo;
our @ISA = qw( Base );
__PACKAGE__->init();
...
package Bar;
our @ISA = qw( Base );
__PACKAGE__->init();