See xsutils.c (part of the source code to Perl itself), which includes a function XS_attributes_boostrap() which was probably produced using XS and then just cut'n'pasted into xsutils.c. Then see this code (in that same file):

void
Perl_boot_core_xsutils(pTHX)
{
    char *file = __FILE__;

    newXS("attributes::bootstrap",    XS_attributes_bootstrap,    file
+);
}
[download]

which defines *attributes::bootstrap to have a Perl code reference that calls the previously named C routine.

BTW, this stuff (for modules that are bundled with Perl) is normally done by putting stuff in the ext subdirectory of the Perl source code tree. For example, see ext/attrs. I guess they didn't want to give people the ability to build Perl without including attributes.xs inside the base Perl run-time.

Update: Yes, you've got it -- and other updates applied (inside bold parens).

        - tye