Ok, color me confused.
Your script as written dones exactly what you claim that it doesn't when I run it (under linux). However, your output fails to expose the flaw in your parsing. The script throws away some of the values. You only capture the value up to the first space, but spaces are a valid character in the value.
I simplified your expression a little. I removed unnessasary backslashes, parens, and quotes and I replaced [^\s] with \S. The new regex is:

%defaults = map {/([^=]+)=(\S+)/?($1=>$2):()} @ARGV;
[download]

Since you already know that (thanks to the shells processing of your arguments) that everything to the right of the equals is a valid part of the value you can correct the problem by simplifying to this:

%defaults = map {/([^=]+)=(.*)/?($1=>$2):()} @ARGV;
[download]

And, I would also simplify the left hand side to be:

%defaults = map {/(.*?)=(.*)/?($1=>$2):()} @ARGV;
[download]

And, actually, you can takes advantage of the fact that calling a regex in list context returns the submatches as a list and use:

%defaults = map {(/(.*?)=(.*)/)} @ARGV;
[download]

Hope this helps

-pete
"Pain heals. Chicks dig scars. Glory lasts forever."