Update:

Example code for obtaining the full array:

use strict;
use warnings;

my @a = (10, 15, -20, -30);
my $highest = abs($a[-1]);
my @full = (0) x ($highest + 1);

# decrement all negative values by 1, otherwise
# we would exclude the end points:
for (@a) {
    $_-- if $_ < 0;
}


my $count = 0;
for (0..$highest) {
    if (abs $a[0] == $_) {
        my $c = shift @a;
        if ($c > 0) {
            $count++;
        } else {
            $count--;
        }
    }
    $full[$_] += $count;
}

print join('', @full), "\n";
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This assumes that there are never two values of same magnitude in the array, ie never exactly matching start- and endpoints. You can generalize it on your own :-)

Second update: I've just realized that the extension is very simple:

Instead of if (abs $a[0] == $_) write while (abs $a[0] == $_), and you're done.

I'll give it a try. BTW, I will also need to sort the array (by abs values) now.

UPDATE: There's no real to use $full[$_] += $count; instead of $full[$_] = $count;, is there? we can therefore also drop the initialization of @full, can't we?

You're right on both accounts.

I just used += because from reading your initial question I wasn't sure if you needed absolute values, or add them to an existing array.