How to ignore specific query parameter with varnish cache?

just strip the _= from the QS.

sub vcl_recv {
  set req.url = regsuball(req.url,"\?_=[^&]+$",""); # strips when QS = "?_=AAA"
  set req.url = regsuball(req.url,"\?_=[^&]+&","?"); # strips when QS = "?_=AAA&foo=bar"

}

keep in mind this will strip the QS - and the QS is not forwarded to the backend. if you need it in the backend you'd need to do in in vcl_hash

beware if you have a custom vcl_hash, to include the default rules, as the custom removes the default hash parameters. http.host || client.ip

VCL hash is the part the generates the cache-uuid

sub vcl_hash {
  set req.http.hash_url = regsuball(req.url,"\?_=[^&]+$",""); # strips when QS = "?_=AAA"
  set req.http.hash_url = regsuball(req.http.hash_url,"\?_=[^&]+&","?"); # strips when QS = "?_=AAA&foo=bar"
  hash_data(req.http.hash_url);
  if (req.http.host) {
    hash_data(req.http.host);
  } else {
    hash_data(server.ip);
  }
}    

this way, the request that produces a MISS also passes _= to the backend. and there is no explicit need for a vmod in such a simple case.