use CGI;

# Parse query string only:
my $cgi = CGI->new($ENV{QUERY_STRING});

# Extract relevant parameters:
my ($query, $params) = map { $cgi->param($_) } qw(query params);

# Upgrade to full CGI context and process uploads:
$cgi = CGI->new(\&upload_handler, undef, 0);

# Handle upload data:
sub upload_handler {
    my ($name, $buffer, $nbytes) = @_;
    # ... do something ...
}