$ perl -Mstrict -Mwarnings -le '
    use Time::Piece;
    my $tp_format = "%Y-%m-%d %H:%M:%S";

    sub get_ms {
        my ($ts, $ms) = split /,/ => shift;
        Time::Piece->strptime($ts, $tp_format)->epoch * 1000 + $ms;
    }

    my $invoke = "2013-08-09 13:00:23,784";
    my $success = "2013-08-09 13:00:23,791";

    print "Time diff: ", get_ms($success) - get_ms($invoke), "ms";
'
Time diff: 7ms