It shouldn't croak for BEGIN.

$ perl -le'use constant BEGIN => 4; print(BEGIN);'
4
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But I'm all for having it croak for INIT.

$ perl -le'use constant INIT => 4; print(INIT);'
Undefined subroutine &IO::File::AUTOLOAD called.
INIT failed--call queue aborted.
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A bug report can be filed.