Does it have to be a regex. It seems straightforward, using "&" and then "=" to split the string.

untested code below:

@pairs = split( /\&/, $ENV{'REQUEST_URI'} );
[download]

Then go through the array, split the strings on '='.

Update:

CrazyInsomniac pointed out that my split was not esacping the '&'

<cite>Just a tongue-tied, twisted, earth-bound misfit. -- Pink Floyd</cite>