OK, I see : the perl comments are included in the C source, and of course a line beginning with a # has unexpected effects there :) So adding a // in front of lines 137 and 164 saves the day:

perl test.pl
1..3
ok 1 - The first 501 primes are found from 1 to 3571.
ok 2 - external_cpp() function gives same results as basic_perl().
ok 3 - inline_c() function gives same results as basic_perl().
# 
# Comparing basic_perl(), external_cpp(), and inline_c() for
# 5 seconds searching 150000 integers.
# 
                Rate   basic_perl external_cpp     inline_c
basic_perl   0.984/s           --         -90%         -96%
external_cpp  9.45/s         860%           --         -58%
inline_c      22.6/s        2197%         139%           --
# 
# I love it when a plan comes together.
#