i don't think there is something to do with regex negative lookahead, the problem lies in the
(\w+(,\s)?)+
it will match "INFO, FILE, SYSLOG", but not "INFO, FILE".
"+" is greedy, it eaten whatever match by "( ... )" . I don't how to make the above regex match less, but i use this
print "has_matched $has\n"    if $has   =~ /^($start(\w+(,\s)?)+?)$(?<!$end)/;
and it will not match $has.