#! perl -slw
use strict;

sub checksum {
	my( $string ) = @_;

	my $chk_sum = 0;
	$chk_sum += $_ for unpack 'C*', substr $string, 0, -5;

	my $twos_comp = hex( substr $string, -5, 4 );
	return ( ( $chk_sum + $twos_comp ) & 0xFFFF ) ? 1 : 0;
}

print "$_ : ", checksum $_ for "\x019999FF1B\x03", "\x019998FF1B\x03";

__END__
P:\test>283701
?9999FF1B? : 0
?9998FF1B? : 1