# old STDIN: 1 2 3
while (<STDIN>) {
   if (some condition) unshift "hi test test" to STDIN
   # new STDIN hi test test 1 2 3 
}

##</code><code>##

while (<STDIN>) {
    if (some condition) $_ = "hi test test " . $_;
    # ...
}

##</code><code>##

# supposing that @ARGV contains file name(s) to be filtered and processed:

open( INP, "my_filter_script.pl @ARGV |" ) or die "can't filter: $!";

while (<INP>) {
    # prefixes and other source code filtering are already done
    # ...
}