Sorry for any confusion it might have caused, but I used the diamond operator simply for convenience. Pretend that instead of <>, I had written <STDIN>, and it might make more sense. How to read the streams wasn't the question I had, however. In fact, the reading program was out of my control anyway. I found it odd that it was able to read two separate streams interactively, and I wondered how to write them both to STDOUT. tye's response clarified that bit for me, specifically where he wrote:

flushing again with CTRL-D means that 0 bytes get flushed to the next read request (and so gets interpretted as EOF).

And toma's response explained how I can emulate the terminal's behavior from a non-interactive perl script. Thanks for taking the time to reply, though.

Update: btw, just to pass along a neat little trick, if using bash (or perhaps other shells support this, but only tested on bash) you could avoid the temporary file in your last example with something like:

    grep "target string" file1                      \
    | cat - <(echo "____STREAM_BOUNDARY____") file2 \
    | script.pl
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