If you assume that a line can only end with a period (.) or a double-quote ("), then the following seems to give the desired output:

#!/usr/bin/perl -w
use strict;

undef $/;

my $text = <DATA>;

$text =~ s/([^."])\n/$1 /g;

print "$text";

__DATA__
this is a
wrapped line.
"This is a wrapped
quote."
this first line is fine.
so is this one.
[download]

Prints:

this is a wrapped line.
"This is a wrapped quote."
this first line is fine.
so is this one.
[download]

Cheers,
Darren :)