There is a Video4Linux module for perl, which you could presumably use to grab frames from 3 BT848 decoder cards, overlay text and output them to 3 different monitors somehow.. the output is probably harder than the input, and Perl is probably not the right language to use for this kind of stuff..
Also, please provide more detail on exactly what you want to do - if its displaying *just* text on 3 computer monitors, then i would simply hook up a budget PC to each monitor running a web server, and set one of the PCs to send messages to the other PCs via HTTP to tell them what to display on their monitor. You could use Perl/TK or even the OpenGL bindings for Perl for this type of thing.
Running 3 monitors off 1 PC can be very tricky, although Windows, MAc and UNIX all support this to some extent.