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;

##</code><code>##

#!/usr/bin/perl

use X;   # X uses Y secondarily

# this works
func_in_X();     # imported into our namespace
Y::func_in_Y();  # fully qualified 
X::func_in_Y();  # fully qualified via namespace X,
                 #   into which func_in_Y() has been exported

# this doesn't work
func_in_Y();     # not imported into our namespace