Oh highly esteemed monks, hear my plea for knowledge.
I am attempting to duplicate in Perl the checksum functionality in the following C code. 
#include <stdio.h>

/*********************************************************************
+***/
/* checksum -- verify checksum                        */
/*********************************************************************
+***/

int checksum (buff, bufflen)
char *buff ;
int bufflen ;
{
   int ctr ;
   int retval ;
   char tmpstr [5] ;
   long strtol () ;
   unsigned int chk_sum = (unsigned) 0 ;
   int twos_comp ;

   ctr = 0 ;
   while (ctr < (bufflen - 5))
   {
      chk_sum = chk_sum + (buff [ctr] & 0x7f) ;
      ctr ++ ;
   }
   strncpy (tmpstr, &buff [bufflen - 5], 4) ;
   tmpstr [4] = '\0' ;
   twos_comp = (int) strtol (tmpstr, (char **) NULL, 16) ;
   retval = (((chk_sum + twos_comp) & 0xFFFF) == 0) ? 0 : 1 ;
   return (retval) ;
}

[download]
Problem? I know ZIP about C, and the definitions I have found for the various functions (all of which appear to be "standard" C ) don't give me enough understanding of what the C function actually does to allow me to replicate the functionality in Perl.
I have gotten as far as the second to last couple of lines, and there I am completely stumped.

Sample input consists of <SOH>9999FF1B<ETX> where <SOH>=0x01 and <ETX>=0x3 respectively.
The inbound is an ASCII string.
Output is either a 0 or 1 (failure / success). I seek either of two things.
1. A plain english description of what the second to the last line of code is actually doing, step by step.
2. An example in Perl of what it is doing. I am quite willing to beat my head against the example until I garner understanding of what is actually happening.

A humble(d) acolyte,
Mike Gucciard

In reply to Conversion of C code to Perl-ese. by gooch

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