Question

It seems some (many?) modules on CPAN are partly implemented in C using XS, and can fall back to a pure-perl implementation if necessary. While this is smart, it can obviously hurt performance, and I would like to know it happens so I can fix the problem.

Is there a general way of stopping or detecting this type of fallback?

For an example of this behaviour take a look at the (very handy) Date::Simple (code snippet)

Answer 1

Any solution would have to be on a per module basis (because the decision on which implementation to use is made by the parent module itself, not some mechanism in Perl). In the case you cited, checking the value of $Date::Simple::NoXs after the use statement will tell you if XS is being used or not.

use Date::Simple;
die "not using XS for Date::Simple\n" if $Date::Simple::NoXs;

For instance, to detect if Scalar::Util is using the XS or pure Perl versions you have to check for the existence of the dualvar function.

use Scalar::Util;
die "not using XS for Scalar::Util\n" unless if @Scalar::Util::EXPORTFAIL;