$config{'db'}{'$key'} = $val;
should be
$config{'db'}{$key} = $val;
but I don't think that will fix your error. The error seems to indicate that $config{'db'} was assigned '' or undef. Are you sure both %config are the same variable?

In reply to Re^3: Can't use string ("") as a HASH ref while "strict refs" by ikegami
in thread Can't use string ("") as a HASH ref while "strict refs" by hesco

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