By and large, don't use glob() in scalar context. The next time your program executes that line, it will provide the next *.ntp file from the original directory, even if $dir has changed, and after getting each *.ntp file, it will return undef once before restarting. Try this instead.

if ( ! defined( $file ) ) {
    $file = ( glob(...) )[0];
    if ( ! defined( $file ) ) {
        die "no NTP file\n";
    }
}
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