Tie::Hash, Tie::StdHash and Tie::ExtraHash are in Tie/Hash.pm, thus use Tie::Hash; is needed even if you want to use the Tie::StdHash class.

>perl -e"use Tie::StdHash qw( );"
Can't locate Tie/StdHash.pm in @INC (@INC contains: c:/Progs/perl588/l
+ib c:/Progs/perl588/site/lib .) at -e line 1.
BEGIN failed--compilation aborted at -e line 1.
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Notice that I didn't actually inherit from Tie::Hash. I didn't inherit from Tie::StdHash either because it doesn't provide any means to store object attributes such as the default value. Tie::ExtraHash does.