This 9th bit is generated internally by the UART based on the value of the parity bit. So, which parity bit is the Perl modules dealing with?

Perl does not do any parity, neither does any other language. The parity bit is generated and checked by the UART hardware. All that the OS and any language on top sees is a parity error if the check fails.

And do the modules allow this definition to be changed at will to drive the address function out onto the RS-485 bus?

That's an application of the mark and space "parity" bits. You could switch the serial format between "mark parity" and "space parity" to set or clear the parity bit from the application.

Of course it is possible to generate and verify the parity bit in the application, that's why some embedded U(S)ARTs allow 9 bit formats (8 bit plus application-specific use of the parity bit). On a generic PC-style USART, the data bits are limited to 8 bit, so if you want to use an application-specific parity bit, you are limited to 4 to 7 data bits.

Alexander