Do a deep copy, not a shallow copy.

package Package2;
use Storable 'dclone';
use base 'Exporter';
use Package1 ':all';

our @EXPORT_OK = ( @{ dclone( \ @Package1::EXPORT_OK ) }, 'foobar' );
our %EXPORT_TAGS = %{ dclone( \ %Package1::EXPORT_TAGS ) };
push @{ $EXPORT_TAGS{all} } => 'foobar';

sub foobar { 'foobar' }

1;
[download]