Answering my own question: I misinterpreted the doc, which is not very clear on this point IMHO.

$ perl -MTime::Moment -E 'while (1) { $x = Time::Moment->now_utc->strf
+time("%FT%T%6f%Z"); say $x if $x !~ /\.\d{6}Z$/ }'
[download]

... prints out a value like "2021-05-01T20:58:09Z" on every complete second it hits.

So I will manually pad with RHS zeroes.