use List::Util qw( sum );
my $hex = '0FB0306F207265616420746869733F';
my $sum = sum map hex,
              map { /../g }
              $hex;
my $sum_as_hex = sprintf '%X', $sum;

##</code><code>##

use List::Util qw( sum );
my $hex = '0FB0306F207265616420746869733F';
my $sum = sum map hex, $hex =~ /../g;
my $sum_as_hex = sprintf '%X', $sum;