[00:53] <lucs> Oh, this works:

[00:53] <lucs> m: my $n = 1; my $s = 'a - a - a a'; $s ~~ s:g/ a /A{$n++}/; say $s;

[00:54] <camelia> rakudo-moar b837303ce: OUTPUT: «A1 - A2 - A3 A4␤»