perldoc comes with the perl package in arch. e.g.
perldoc perllol. For most modules all the utility does is build the documentation from the POD in the source though.
/usr/lib/perl5/core_perl/POSIX.pm happens to have no POD so doesn't build any documentation. I guess because it is an XS module so most of it's function is in C somewhere else.
Comparing Arch to a Debian box with perldoc, on Debian some of the core modules also have a separate
.pod file in
/usr/share/perl/5.x/ whereas Arch has none.
POSIX.pm seems to be slightly different again in that the
.pod is in the lib directory..
debian$ find /usr/lib/ -name "*.pod"
Which makes it look like debian does something custom here.
In fact, if you check the perl source
POSIX.pod is a separate file sitting next to the perl module.
Maybe file a bug with arch to have it included? I guess you could argue for it to be changed in Perl instead, if it goes against some standard. There's Perl people on stackoverflow that would probably know.