Using POSIX::strftime() is certainly the way to go... but here's what your get_timestamp() could have looked like — just in case you want to learn from what you did wrong.

sub get_timestamp {
    my ($sec,$min,$hour,$mday,$mon,$year) = localtime(time);

    return sprintf('%u-%02u-%02u %02u:%02u:%02u.000',
                   $year+1900, $mon+1, $mday, $hour, $min, $sec);
}
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