use strict;
use warnings;

my @payloads = qw(
00010006523200060013436973636f203132303030475352
00010006523200060012436973636f3132303030475352
00010006523200060013436973636f20313230303047535284
00010006523200060012436973636f313230303047535284
);

for (@payloads) {
    prepchksum(pack "H*", $_);
    print "\n"
}

sub prepchksum {
    my ($payload) = @_;

###DEBUG: 
printf "BEFORE = %s\n", (CORE::unpack "H*", $payload);
   if (length($payload)%2) {
      my $padded = substr $payload, 0, -1;
      $padded .= "\0";
      $padded .= substr $payload, -1;
###DEBUG: 
printf "MID1   = %s\n", (CORE::unpack "H*", $padded);
      # Compensate off-by-one error
      if (ord(substr $padded, -1) >= 128) {
         my $end    = ord(substr $padded, -2, 1);
         my $endend = ord(substr $padded, -1);
         $end--;
         $endend--;
###DEBUG: 
printf "CMPSAT = %i %i\n", $end, $endend;
         # if 0 before --, then -1 results in warning to pack below
         # -1 is in essense \xff or 255
         if ($end == -1) { $end = 255 }
         if ($endend == -1) { $endend = 255 }

         $padded = substr $padded, 0, -2;
         $padded .= CORE::pack('CC', $end, $endend);
      }
      $payload = $padded;
   }
###DEBUG: 
printf "AFTER  = %s\n", (CORE::unpack "H*", $payload);
}