Increment loop index within loop body

Hmm.

The best way is probably to not use an explicit loop. Just calculate the sequence directly.

# the actual sequence logic
my @seq = grep *.is-prime, (42, { .is-prime ?? $_+<1 !! $_+1 } … *);

# display code
say (1+$_).fmt("%-4s"), @seq[$_].flip.comb(3).join(',').flip.fmt("%20s") for ^42;

Output:

1                     43
2                     89
3                    179
4                    359
5                    719
6                  1,439
7                  2,879
8                  5,779
9                 11,579
10                23,159
11                46,327
12                92,657
13               185,323
14               370,661
15               741,337
16             1,482,707
17             2,965,421
18             5,930,887
19            11,861,791
20            23,723,597
21            47,447,201
22            94,894,427
23           189,788,857
24           379,577,741
25           759,155,483
26         1,518,310,967
27         3,036,621,941
28         6,073,243,889
29        12,146,487,779
30        24,292,975,649
31        48,585,951,311
32        97,171,902,629
33       194,343,805,267
34       388,687,610,539
35       777,375,221,081
36     1,554,750,442,183
37     3,109,500,884,389
38     6,219,001,768,781
39    12,438,003,537,571
40    24,876,007,075,181
41    49,752,014,150,467
42    99,504,028,301,131