package PkgA;
use List::Util qw( first );
BEGIN { our @ISA = qw( ... ); }
sub foo { 'Whee!' }
sub check { return first { $_->can('foo') } @ISA; }

##</code><code>##

package PkgA;
BEGIN { our @ISA = qw( ... ); }
my $overrides_foo;
BEGIN { $overrides_foo = __PACKAGE__->can('foo'); }
sub foo { 'Whee!' }
sub check { return $overrides_foo; }