%step = ( 20 => 1, 50+1 => -1 );

##</code><code>##

my %step;
foreach(@range) {
    $step{ $_->[0] }++;
    $step{ $_->[1]+1 }--;
}

##</code><code>##

my @edge = sort { $a <=> $b } keys %step;
my $step = 0;
my $x = shift @edge;
for(my $i = $x; defined $x; $i++) {
    if($i == $x) {
        $step += $step{$x};
        $x = shift @edge;
    }
    $arr[$i] += $step;
}

##</code><code>##

@arr = (0) x 120;
@range = ([20, 50], [30, 100]);

##</code><code>##

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0