#a.pl
use bspace;
use cspace;
bgo();
cgo();

#bspace.pm
package bspace;
use Exporter;
use cspace;
@ISA='Exporter';
@EXPORT=qw( bgo bhelp );
sub bgo { chelp(); }
sub bhelp { print "bhelp\n"; }
1;

#cspace.pm
package cspace;
use Exporter;
use bspace;
@ISA='Exporter';
@EXPORT=qw( cgo chelp );
sub cgo { bhelp(); }
sub chelp { print "chelp\n"; }
1;

##</code><code>##

a.pl line 1 compiled: "use bspace"
bspace line 1 compiled: package bspace;
bspace line 2 compiled: use Exporter;
Exporter.pm compiled
main code of Exporter.pm executed
Exporter::->import called (uselessly)
bspace line 3 compiled: use cspace;
cspace line 1 compiled: package cspace;
cspace line 2 compiled: use Exporter;
Exporter::import called (uselessly)
cspace line 3 compiled: use bspace;
bspace::->import called (but doesn't yet exist since bspace::ISA is not yet set to inherit import() from Exporter, so bgo and bhelp are not imported into cspace)
cspace line 4 compiled but not executed: @ISA = 'Exporter'
cspace line 5 compiled but not executed: @EXPORT = qw(cgo chelp);
cspace line 6 compiled: sub cgo { bhelp(); }
cspace line 7 compiled: sub chelp { print "chelp\n"; }
cspace line 8 compiled but not executed: 1;
cspace lines 4,5,8 executed
cspace::->import called: cgo and chelp are exported into bspace
bspace line 4 compiled but not executed: @ISA = 'Exporter'
bspace line 5 compiled but not executed: @EXPORT = qw(bgo bhelp);
bspace line 6 compiled: sub bgo { chelp(); }
bspace line 7 compiled: sub bhelp { print "bhelp\n"; }
bspace line 8 compiled but not executed: 1;
bspace lines 4,5,8 executed
bspace::->import called: bgo and bhelp are exported into main
a.pl line 2 compiled: use cspace
cspace::->import called: cgo and chelp are exported into main
a.pl line 3 compiled but not executed: bgo()
a.pl line 4 compiled but not executed: cgo()
a.pl line 3 executed: bgo() (imported from bspace)
bspace::bgo calls chelp() (imported from cspace)
cspace::chelp prints "chelp\n"
a.pl line 4 executed: cgo() (imported from cspace)
cspace::cgo calls bhelp() which was not imported from bspace and so doesn't exist.

##</code><code>##

package bspace;
use strict;
use warnings;
use Exporter (); # () avoids (harmless but) useless call to Exporter::import;
BEGIN {
    our @ISA = 'Exporter';
    our @EXPORT = qw(bgo bhelp);
}

##</code><code>##

# A.pm
package A;
use strict;
use warnings;
use Exporter ();
BEGIN { our @ISA = 'Exporter'; our @EXPORT = 'foo' }
sub foo { print "foo" }
# and in the same file A.pm
package A::B;
use A;
sub bar { foo() } # without the BEGIN block above, the use A won't have imported foo into A::B