use constant N => 1;         # for example
use constant M => 10_000;    # for example
my $sum;

# Method 1: a simple for loop.
$sum = 0;
for (my $i = N; $i <= M; $i++) {
    $sum += $i;
}

# Method 2: an evil use of map by throwing away its result.
$sum = 0;
map { $sum += $_ } N..M;

# Method 3: the APL way
$sum = eval join('+', N..M);