I am unable to reproduce your result.

$ cat ct.pl
#!/usr/bin/perl 
use strict;
use warnings;
use POSIX qw( :unistd_h :sys_wait_h setsid WNOHANG );
POSIX::setsid() or die 'failed';
$ perl -cw ct.pl
ct.pl syntax OK
$ perl ct.pl
$ perlcritic --stern ct.pl
ct.pl source OK
[download]

This is with POSIX 1.38_03 and Perl::Critic 1.125.

What do the wise folks here think of Perl::Critic generally?

It's a useful tool both for the individual and for teams to apply consistent policies to code. No more or less than that.