#!/usr/bin/perl -w
use strict;
use vars qw(@ISA @EXPORT);

package A;

BEGIN
{
    require Exporter;
    @A::ISA    = 'Exporter';
    @A::EXPORT = qw(CA sa);
}

use constant CA => 1;

sub sa
{
    my $a = CA;    # fine
}

package B;

BEGIN
{
    A->import(':DEFAULT');
}

@B::ISA = 'A';

sub sb
{
    my $b = CA();    # fine
    my $c = CA;      # fine, but only after import.
}