# 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