You're really close. The problem is that \S+ matches anything besides whitespace, including colons and pipes. By default, + is greedy, so it matches as much as it can. So the first \S+ matches monsa::clear::1|red::23|blue::50|monsb::---quite a bit more than you want it to!

The two solutions are to not match colons or pipes with your regex, by using [^:|]+ instead of \S+, or to tell the regex to match non-greedily, by saying \S+? instead of \S+. I've done the latter here, and it seems to give the output that you want:

#!/usr/bin/perl

$_="monsa::clear::1|red::23|blue::50|monsb::clear::80|red::90|blue::10
+0|";

@instances = m/(\S+?::\S+?::\S+?\|\S+?::\S+?\|\S+?::\S+?\b)/g;
foreach $inst (@instances) {
   print "found $inst\n";
}
exit;
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