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unless ($file_in =~ /^[\w][\w\._-]*$/) {
    print "Insecure file_in\n";
    exit;
}

if ($file_out =~ /^([\w][\w\._-]*)$/) {
    $file_out = $1;
}
...
    print "Insecure file_out\n";
    exit;
}