We really need more information about what this program is actually doing, and how the C code relates to the Perl code. Your no-op example are precisely this ... no-ops ... and could only possibly be exercising the thunking logic that passes control to the inline environment and back. Could it be that the two Perl interpreters are compiled differently? "C" optimization can't possibly be a factor when the "C" subroutines don't do anything: the difference must be in the two Perl interpreters themselves.