Although, as LanX already noticed, my descriptions can contain several paragraphs, thank you for suggesting paragraph mode. Your solution is both simple and elegant.

And the technique with encoding line breaks could indeed work. I would switch the two regexps, so there is no need to assign the capture value to $key, and I would use the \\ linebreak symbol, as it is already associated with manual linebreak (in LaTeX).

#!/usr/bin/perl
use strict;
use warnings;

my %desc;
{
    local $/ = "";
    while (<DATA>) {
        s/\\\\/\n/;
        s/^(.*)\n//;
        $desc{$1} = $_;
    }
}

print "{$_} => \n$desc{$_}" for (keys %desc);

__DATA__
house_west
You are standing in an open field west of a white house,
with a boarded front door.
\\
There is a small mailbox here.

house_south
You are facing the south side of a white house. 
There is no door here, and all the windows are boarded.

house_behind
You are behind the white house.
A path leads into the forest to the east. 
\\
In one corner of the house there is a small window which is slightly a
+jar.
[download]

Again, thank you for your suggestion and the provided examples.