There are so many free, good quality, general purpose database products around I have to wonder why you would want to write another - unless you have some ground breaking search or index algorithim. Most database engines are written in C, for performance reasons, and may even have assembler in some performance critcal areas.

Perl is used as an interface because most database engines have a C interface, and a wrapper can be written using the Perl-C API (that is XS). Many other languages use C wrappers in this way, not just Perl.