my $max = 100000000;
my @list = (
    [ 39887, 399900 ],
    [ 23, 900 ],
    [ 8000, 10000 ],
    [ 50000, 1000500 ],
);

###

@list = sort { $a->[0] <=> $b->[0] } @list;

@res = (0);
$last = shift @list;

for (@list) {
    if ($_->[0] > $last->[1] + 1) {
        push @res, $last->[0] - 1, $last->[1] + 1;
        $last = $_;
    } else {
        $last->[1] = $_->[1] if $_->[1] > $last->[1];
    }
}

push @res, $last->[0] - 1, $last->[1] + 1;

$res->[-1] < $max ? push @res, $max : pop @res;

for (0 .. $#res) {
    print $res[$_], $_ % 2 ? "\n" : '-';
}