Here's one possible regex approach using named captures (perlre) and %+. However, it's starting to get a little complex, so the line-based state machine type approach from Corion's post is starting to look a little better in this case (I gave some more examples of the state machine approach in this thread).

use warnings;
use strict;
use Data::Dump;

local $/ = "\n\n\n";
while (<DATA>) {
    # clobber the header
    s/ ^ \s* Dumpdata\s+example \s* \n \s* -+ \s* \n //msx
        or next;
    my %row;
    $row{$+{key}} = $+{val}
        while m{
            (?: # the very first key doesn't need colon
                  \A \s* (?<key> \w+   )
                # but the other keys need colons
                | ^  \s* (?<key> \w+ : )
            ) \s+
            # values should end at the next key
            (?<val> (?: (?!^\s*\w+:) . )+ )
        }xmsg;
    s/\s+/ /g for values %row;
    dd \%row;
}

__DATA__

Dumpdata example
-----------------

Warning bad news here 
    Detail:              Some really nice infos these are 
    Info:              This is a problem
                       but there is a solution


Dumpdata example
-----------------
Warning test
    Detail: foo
            bar
    Info:   quz
            baz
    Spec:   blah
[download]

Output:

{
  "Detail:" => "Some really nice infos these are ",
  "Info:"   => "This is a problem but there is a solution ",
  "Warning" => "bad news here ",
}
{
  "Detail:" => "foo bar ",
  "Info:"   => "quz baz ",
  "Spec:"   => "blah ",
  "Warning" => "test ",
}
[download]

Update: Using the "branch reset" pattern (?|...) allows for a little bit of simplification:

    my %row = m{
            (?| \A \s* ( \w+   )
              | ^  \s* ( \w+ : ) ) \s+
            ( (?: (?!^\s*\w+:) . )+ )
        }xmsg;
[download]