I would bet that for the same reason that:

perl -Mstrict -we 'my $hash={foo => 1}; print $hash->{"foo"}->{"bar"}'
[download]

prints the same thing. Basically %hash->{$base} does not point to a HASH ref, so when you try to use it as one strict points it out, and as davido points out (as well as -w or warnings): Using a hash as a reference is depreciated.

-enlil