"dude" do you know what fork means? It launches a new process, except in C it will copy over the file descriptor table and vm area and all that. If you're looking for the exact same fork() call in tcl, you're not going to find it. Your options are open/exec, which does the same thing as fork except doesn't copy over the file descriptors and vm area.
Also, you said "file & in bash" which led me to believe this is a stand-alone script or something that you want executed in the background, and thus open/exec is the solution. If that's not what you meant, why don't you try explaining it again and use more words.