Caveats apply. This is not recursive and it almost certainly should be; it fails on deeper data structures. It only serializes. I am almost positive there is something in some corner of the CPAN that does this but I couldn't find it in a somewhat casual search. This is first draft quality code. This style of query string is popular with PHP and Ruby apps. I think this is how jQuery does it if you don't request "traditional" (meaning *standards compliant* serialization) so it might be good to just copy its code if there turns out to be nothing solid in Perl already.

use strictures;
use Carp;
use URI::Escape;

my %hash = (  "name"     => "test name",
              "file_ids" => [ 1, 2 ],
              "sub"      => { "name" => "foo",
                              "message" => "bar" } );

print serialize_web2_0_query_string(\%hash), $/;

sub serialize_web2_0_query_string {
    no warnings "uninitialized";
    my $datum = shift;
    my $strings = shift || [];
    for my $key ( keys %$datum )
    {
        if ( ref $datum->{$key} eq "HASH" )
        {
            while ( my ( $k, $v ) = each %{ $datum->{$key} } )
            {
                push @$strings, sprintf "%s[%s]=%s", uri_escape($key),
+ uri_escape($k), uri_escape($v);
            }
        }
        elsif ( ref $datum->{$key} eq "ARRAY" )
        {
            push @$strings, sprintf "%s[]=%s", uri_escape($key), uri_e
+scape($_) for @{ $datum->{$key} };
        }
        elsif ( not ref $datum->{$key} )
        {
            push @$strings, join "=", map uri_escape($_), $key, $datum
+->{$key};
        }
        else
        {
            croak "Nope. Don't know what to do with ", $key, " => ", $
+datum->{$key};
        }
    }
    return join "&", @$strings
}
__DATA__
sub[name]=foo&sub[message]=bar&file_ids[]=1&file_ids[]=2&name=test%20n
+ame
[download]