#! perl -slw
use strict;

my $test = 'xxx,xyyy,zzz,zzz,ppp,qqq,rrr';

my $re = qr[
    (?<!zzz),(?!zzz)  # a comma neither preceeded nor followed by the token.
    |
    (?<=zzz),(?!zzz)  # or predeeded but not followed
    |
    (?<!zzz),(?=zzz)  # or not preceeded but followed.
]x;

print for split $re, $test;

__END__
P:\test>junk
xxx
xyyy
zzz,zzz
ppp
qqq
rrr