This isn't addressing specifically how Perl is compiled, but rather how Perl implements the notion of characters: Because of Perl's Unicode implementation, a Perl character isn't necessarily a single byte. Consider the following:

binmode STDOUT, ':encoding(utf8)';
my $cp = "\x{3a3}";
print "$cp\n"; # Prints an upper case sigma.
print ord( $cp ), "\n"; # prints 931.
print 0x00FF & ord( $cp ), "\n"; # prints 163.
if( ord( $cp ) > 255 ) {
    print "Wide character.\n";
}
if( ord( $cp ) != ( 0x00FF & ord( $cp ) ) ) {
    print "Wide character.\n";
}
[download]

If you're not dropping into XS, what application is there for getting at the actual value of CHAR_BIT during compilation? (Just to satisfy my own curiosity, and to learn something new).

Update: To be more clear in my final question: I do understand that the C implementation could possibly be compiled with CHAR_BIT != 8. We see checks for it within the XS code of Scalar::Vec::Util and Math::MPC, for example. But what I'm curious about is how it makes any difference from the perspective of a Perl script that doesn't use XS (aside from the potential that some XS modules will break because they're not checking like they should).