Thanks for the input, I now have something that works. Using ord was very helpful, I have also modified it to use sysopen and sysseek, for some reason using sysread with the offset parameter doesn't work for me hence the sysseek. As for using CRCs, I know the checksum is weak but I have just been tasked to automate an existing process, not to change the process so I don't get to choose the method. I will look at the modulo arithmetic approach to see if I can get that working rather than the mess of packing and unpacking. 
sub Checksum {
    my @Arguments = @_ ;
    my $Start = $Arguments[0] ;
    my $Size = $Arguments[1] ;
    my $FileName = $Arguments[2] ;
    
    my $FileHandle ;    
    unless ( sysopen $FileHandle, $FileName, 0 ) {
        die "\nsysopen failed";
    }
    
    my $BufferByte = 0 ;
    my $Checksum = 0 ;
    my $ChecksumLoopCounter = 0 ;
    my $ChecksumByteOffset = $Start ;
    unless ( sysseek($FileHandle, $Start, 0) == $Start ) {
        die "\nsysseek error" ;
    }
    while ( $ChecksumLoopCounter < $Size ) {
        unless ( sysread ( $FileHandle, $BufferByte, 1) == 1 ) {
            die "sysread size error" ;
        }
        my $Byte = ord($BufferByte);
        $Checksum = unpack("S" , pack("S", $Checksum + $Byte));
        $ChecksumLoopCounter++ ;
        $ChecksumByteOffset++ ;
    }
    close $FileHandle ;
    return $Checksum;
}

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In reply to Re: Accessing individual bytes of a binary file as numerical values by Chris01234
in thread Accessing individual bytes of a binary file as numerical values by Chris01234

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