The "exec" builtin in bash (sh on Linux seems to be too) will run the perl process without creating a new process by replacing the original shell process, just like the C exec* family of functions.

So you ought to be able to run a script by "./script.sh" to create a new process and then "exec perl script.pl" to continue execution without introducing a new process, but the original shell script will terminate (control will not return to the shell script).