package X;

require Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw(func_in_X);

use Y;

sub func_in_X {
    print "func_in_X called.\n";
}
1;

---

package Y;

require Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw(func_in_Y);

sub func_in_Y {
    print "func_in_Y called.\n";
}
1;