Both the other replies look like they'll work! I'd advise taking either of them, and turning them into a function that takes in the text to encode and the maximum length, and either returns the correctly-encoded string, or does a die on error. Something like (modifying jwkrahn's version):

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);
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The benefit of making a function is it's more widely reusable, and is generally better practice as one can write (and test) each piece of a program bit by bit. Edit: and creating stx_decode would be an obvious step, and then you'd have two little functions that nicely wrap that format.