$string = "a:b::c   d";
@patterns = (':','::','\s+');
my $re = join '|', sort { length $b <=> length $a } @patterns;
@fields = split /$re/, $string;
print "Got '$_'\n" for @fields;

__DATA__
Got 'a'
Got 'b'
Got 'c'
Got 'd'