#before loop
$cntdiff = new Math::BigInt '0';
$predefbi =  new Math::BigInt '0';
# inside read loop
# for example
$firststr = "ffffccbbaa0";
$laststr = "ffffccbbaa9";

# Make sure it is clear that its hex...
$firststr = "0x".$firststr;
$laststr = "0x".$laststr;

$strtcnt = new Math::BigInt $firststr;
$endcnt = new Math::BigInt $laststr;

$cntdiff = $endcnt - $strtcnt;

# the above works OK but if I try something like

$predefbi = $firststr;

# it doesn't seem to work