Do you mean you wish to ignore all lines which either have only the string "case :" or start with a colon or start with the string "struct" and put that in one regex?

If so, this will work:

use Modern::Perl;

while (<DATA>) {
    next if /^\s*case :$|^:|^struct/;
    print;
}

__DATA__

case :
   case :
case : keep me
: ignore this
keep this line
struct: get rid of me
 struct: but keep me
[download]

By the way, your regex will also ignore lines which have anything between "case" and the colon. That does not seem to conform to your requirement.