use URI::Split qw(uri_split);

my $uri = 'http://example.net/?foo=bar&baz=fu';
my ($scheme, $auth, $path, $query, $frag) = uri_split($uri);
my @sorted_query = sort split /&/, $query;
$query = join '&', @sorted_query;

my $new_uri_string = $scheme . $auth . $path . $query . $frag;