field_filters => {
  my_filter => [ \&MyFilter ],
},
[download]

or...

field_filters => {
  my_filters => [ sub { ...code here... } ],
},
[download]

Or, as a variation on the latter one:

field_filters => {
  my_filters => [ sub { MyFilter(@_) } ],
},
[download]

The first one is great if you have a named sub to pass in anyway. But I like the second one since I'm creating a sub solely for passing around as an argument like this anyway.