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

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

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

...
    # prefixes and other source code filtering are already done
    # ...
}