How to get current line number during "raku -n" iteration?

Use a state variable for it and increment that. Most conveniently, there is the anonymous state variable $, meaning one can do:

echo -e "foo\nbar\nbaz" | perl6 -ne 'say $++ ~ ": $_"'

Which gives:

0: foo
1: bar
2: baz

Use ++$ instead for base-1 numbering. There isn't anything special built in to the handle; it falls neatly out of state variables or, for an explicit iteration, .lines.kv, and then it's only paid for by things that want it.

The LN module is probably what you want to use.