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

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