TIMTOWDI, and assuming, your string need not be checked against containing only legal characters:

my $string = '---LL--C----LCSH-------CSHL-------LCSLH-------LCCHLSHCL-
+---';
foreach (split /L*-+L*/, $string) {
        print $_,"\n" if length($_)>2;
}
[download]

Update: I just noticed: This will fail if the string starts or ends with "L" and not with "-" or anything else. So making the $string in the split a "-$string-" is one workaround.