I'd be surprised if a non-public-not-for-general-use internal implementation bits were documented, esp when so much of the public-for-general-use bits are under documented

http://perldoc.perl.org/perlguts.html#Putting-a-C-value-on-Perl-stack

http://search.cpan.org/dist/illguts/index.html

http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=define%20XPUSHi

http://perl5.git.perl.org/perl.git/blob?f=pp.h#l302

 #define XPUSHi(i)       STMT_START { sv_setiv(TARG, (IV)(i)); XPUSHTARG; } STMT_END

http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=define%20STMT_START

 #   define STMT_START   (void)( /* gcc supports "({ STATEMENTS; })" */