DB_File has contained this code longer than perl 5.5 has existed. The situation you describe is nigh impossible (bless would have to fail and return 1, TIEHASH would have to fail and return 1).

15 May 1997 http://search.cpan.org/~chips/perl5.004/

07 May 1997 http://cpansearch.perl.org/src/PMQS/DB_File-1.14/DB_File.pm

sub TIEHASH
{
    my $pkg = shift ;
 
    bless { VALID => { map {$_, 1} 
               qw( bsize ffactor nelem cachesize hash lorder)
             }, 
        GOT   => {}
          }, $pkg ;
}
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If you intentionally return 1 , tied would return undef.