You could always use [\~] to return all of the intermediate results. Maybe

    my $s;
    for [\~] 1..* {
        last if .chars >= 1_000_000;
        $s := $_;
    }
[download]

though it's not terribly functional to use temporary bindings like that. If we made optional parameters not "eat" values from the list then we could do a lookahead:

    my $s = first gather for [\~] 1..* -> $x, $y? {
        take $x if $y.chars >= 1_000_000;
    }
[download]

or some such...