Grandfather, you may have hit a bug. I copied your code and ran it without errors (Linux machine), this certainly looks as if to depend on the environment/versions:

Test::More::VERSION: 0.47
DBI::VERSION: 1.40
Perl version: 5.8.5
ok 1 - after fetchall_hashref
1..1
[download]

The error Attempt to free unreferenced scalar suggests that Perl tries to decrement the reference count of a scalar to find the unpleasant surprise that it has already reached 0 and in fact the variable should have been released before. Without some XS code such an error is tough to get!