my $limit_3 = param("limit_3");
my $limit = param("limit");
my $add_limit = ($limit_3 + $limit);
my $limit = ($limit_3 + 0);

print qq($limit);

<form method="POST" action="add_to_the_number.pl"><input type="hidden" name="limit_3" value="10"><input type="hidden" name="limit" value="$limit"><input type="submit" name="action" value="Next"></form>