use strict;
use warnings;

sub myMessage {
    my $payload = shift;
    my $format
        = "\N{STX}"
        . '%08u'    # Byte count
        . '0' x 8   # Reserved
        . '%s'      # Pay load
        . "\N{ETX}"
    ;
    return sprintf $format, 18 + length($payload), $payload;
}

my $result = myMessage("<PAYLOAD>");

print $result, "\n";
print unpack 'H*', $result;

##</code><code>##

>perl CuriousMark.pl
.0000002700000000<PAYLOAD>.
02303030303030323730303030303030303c5041594c4f41443e03
>