$ENV{REQUEST_METHOD} = "POST";

open F, "testplan.txt" or die $!;
while ( chomp ( my $data = <F> ) ) {

  ### DO ANYTHING TO MAKE $data BECOME STDIN ###  

  # the main() script that reads and handle the form
  # print final result
  # next test plan data
}
close F;
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You could close STDIN and re-open it as a normal filehandle:

close(STDIN);
open(STDIN, 'foo');
print $_ foreach(<STDIN>);
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The documentation for 'open' shows how to save and restore filehandles, and some other dirty tricks you can play.