use constant MaxMessageLength => 300_000;

sub stx_encode {
  my ($Message, $max_length) = @_;
  my $MsgLgth = 18 + length $Message;
  die "Formatted message exceeds MaxMessageLength" if $MsgLgth >= $max_length;
  return sprintf "\x2%08d00000000%s\x3", $MsgLgth, $Message;
}

print stx_encode('hello', MaxMessageLength);