A recursive search does the trick so long as run time on typical data is not an issue:

use strict;
use warnings;

my @parts;

while (<DATA>) {
    my ($start, $end) = /(\d+)\D(\d+)/;

    next if !defined $end;
    push @parts, [$start, $end];
}

@parts = sort {$a->[0] <=> $b->[0] || $a->[1] <=> $b->[1]} @parts;
print join ('-', @$_), "\n" for trailsBetween(0, 2000, @parts);


sub trailsBetween {
    my ($prevEnd, $to, @parts) = @_;
    my @tails;

    return [$prevEnd] if $prevEnd == $to;

    while (@parts) {
        my ($start, $end) = @{shift @parts};

        last if $start > $prevEnd;
        next if $end < $prevEnd;
        push @tails, trailsBetween($end, $to, @parts);
    }

    unshift @$_, $prevEnd for @tails;
    return @tails;
}


__DATA__
0-200
0-1800
200-400
400-600
600-800
800-1000
1000-2000
[download]

Prints:

0-200-1800-2000
0-200-400-600-800-1000-2000
0-1800-2000
[download]