The problem of dealing with readonly inputs is a wide-spread one with XS code. Win32::API demonstrates this. In order to ensure against traps when passing parameters from Perl that are destined for system APIs written in C that will expect to be able to use strlen to determine string lengths, the author chooses to append a null to string inputs. In most cases this is unneccessary as Perl already does this, but given the unknowable nature of the APIs called through that interface, and the parameters they can take, it's good insurance.

The downside is that even when passing string constants that are never modified, it forces the user to do

my $stringConstant = 'the constant';
$api->Call( $stringConstant, ... );
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In order that the module can append the insurance null. This is a necessary precaution, but a PITA for the user. I wish to avoid that scenario. Creamygoodness has explained the cause of the problem I was encountering, so my post has equiped me with the information I need to deal with this situation correctly.

The problem is a generic one with all XS code, and as I am not under time pressure to provide the solution, it is my nature to try and understand the nitty gritty of the problems I encounter so that I will be equiped to deal with them in the future. I'm a strong proponent of YAGNI when designing code, but in this case, the problem is generic enough that I will definitely need it at some point in the future, even if I decided that it isn't critical to the project that raised it.