#!/usr/bin/perl

# http://perlmonks.org/?node_id=1201076

use strict;
use warnings;

print +(join '', <DATA>) =~ s/^ .*\K\n (?=.*\n-)//gmr;

__DATA__
a data line
 Test123
 Blah
-
something else
 do not change this line
-
a data line
 combine th
 is line
-
something else