Some systems (e.g. Linux) do not allow suid scripts. You can set the suid bit on the file, but the system will just ignore it and run the script without switching users. The reasoning is that shell scripts are very insecure and allowing them to work results in more harm than good.

The main thing that suidperl did was allow suid Perl scripts to work even on systems that operate this way.

There is always the danger of a user trying to escape to a shell. That is one of the big reasons that suid shell scripts are disallowed on those systems. The idea behind taint mode is that it will help the programmer to reduce that risk by keeping track of things that come from user input. If the program never uses any input, it can't do anything unexpected (but it might not be able to do anything useful either).