Not certain if it'd be faster, and you might consider it uglier, but...

Have you considered just using pack() in Perl to turn the Perl array into an int[], then passing the resulting binary scalar into your XS, grabbing the SvPV off the scalar, and treating it like an int[] ? That leaves most of the heavy lifting to Perl.

BTW: In your version, I fear you've got a memory leak unless foo_in_c() free()'s your temp array...