If there's many more numbers than intervals, iterating over the number is indeed a bad idea. But if I understand your demand correctly, the desired can be achieved by iterating over intervals (or rather, over their min and max values: 2..5 and 4..7 is the same as 2..7 and 4..5 regarding coverage).

So you would have to create two arrays (e.g. @min and @max) holding respectively the lower and higher bounds of all intervals. Then:

my $count = 0;
foreach $min (@min) {
    $count++ if ($_[0] >= $min)
}
foreach $max (@max) {
    $count-- if ($_[0] > $max)
}
return $count;
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Would give the coverage of $_[0]. Of course, it can be optimized further by sorting @min and @max beforehand and using dichotomy, which would get you an O(ln(X)).