Bash and Perl

Well, $() is POSIX. And "local" has been proposed for inclusion in POSIX sh and found to be generally available, in some form, in most sh implementations already.

As others have pointed out, if you want to write bash scripts that's fine and it's trivial to do: just start your script with #!/bin/bash. If you want to write POSIX shell scripts, start your script with #!/bin/sh. If you use #!/bin/sh and you use bash-isms, your script is simply wrong.