more self answering here

if ($opt_f eq '') {
    print "Please enter the OUTPUT filename [defaults to screen]: ";
    $output_filename = <STDIN>;    
    chomp($output_filename);
    if ($output_filename eq '') {

    } else {
        open(SAVOUT, ">&STDOUT"); # save the stdout fhandle
        open(STDOUT,">" . $output_filename);
        select(STDOUT);  # force a flush on stdout so that any output 
+will be in sync.
        $| = 1;
    }
} elsif ($opt_f ne "DEFAULT") {
    $output_filename = $opt_f;
}
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unfortunately, it dosen't seem to work :(

Originally posted as a Categorized Answer.