So why not just something like

Win8 Strawberry 5.8.9.5 (32)  Tue 07/19/2022  0:41:07
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Data::Dump qw(dd);

use constant MaxMessageLength => 300_000;

my $Message = 'how now brown cow';
my $MsgLgth = length($Message)+18;

$MsgLgth <= MaxMessageLength
    or die sprintf 'Formatted Message len %ld exceeds MaxMessageLength
+ %ld',
                    $MsgLgth, MaxMessageLength
                    ;

$Message = sprintf "\x02%08d00000000%s\x03", $MsgLgth, $Message;
dd $Message;

^Z
"\x020000003500000000how now brown cow\3"
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