I understand this more fully now after sleeping over it and after your excellent explanation and alternative with use Parent; @ISA = 'Parent'. (But — in the single file case — realize I cannot use because that will call require which will try to read Parent from a file. In turn, use base apparently doesn't do this if it notices the package is already there — not relying only on %INC.)

With respect to the unwanted exportation of the constants, it might be better to implement this as suggested by jettero (Re^2: Constants and Subclasses...) and do:

our %EXPORT_TAGS = (
     CONSTANTS => [qw(FOO)], # and other constants
);
our @EXPORT_OK;
Exporter::export_ok_tags('CONSTANTS'); # populate the @EXPORT_OK
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and then

BEGIN { P->import(':CONSTANTS'); }
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As the constants are not exported by default anymore, the user of the code should explicitly call for them or not.

Yeah, jettero, you were right from the beginning about the convenience of @EXPORT_OK and %EXPORT_TAGS.