A small clarification as to what happens here: The string 'END' and the __C__ section are combined to produce the C unit (C source code). After preprocessing, C compiler will see it as

END

void boot_duh(PerlInterpreter* my_perl __attribute__((unused)), CV* cv
+ __attribute__((unused)));
void boot_duh(PerlInterpreter* my_perl __attribute__((unused)), CV* cv
+ __attribute__((unused)))
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Resulting bogus code trips up the compiler.

Now the question is, why such a feature? Is it used a lot? Shouldn't there be a warning when source string and section are both present? Hm.