Better to bring in the extra text from a new source,

while (<STDIN>) {
    $_ = "hi test text" if some_condition;
    # etc
}
[download]

Any chance you'll post your forth interpreter here? I'd like to see it.

Update: One way to slip in an extra STDIN is to localize the handle:

while (<STDIN>) {
    do {
        local $_ = $_;
        open local(*STDIN), @other_args or die $!;
        # do stuff
    } if some_condition;
    # more stuff
}
[download]

That takes care of throwing away $_, too. BTW, your forth interpreter is 404 for me.