Perl6::Form sprintf printf doesn't show example so look for one in the tests 
1170 
1171 {
1172     # gh #17221
1173     my ($off1, $off2);
1174     my $x = eval { sprintf "%n0%n\x{100}", $off1, $off2 };
1175     is($@, "", "no exception");
1176     is($x, "0\x{100}", "reasonable result");
1177     is($off1, 0, "offset at start");
1178     is($off2, 1, "offset after 0");
1179 }
1180

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https://perl5.git.perl.org/perl5.git/blob/HEAD:/t/op/sprintf2.t#l1174 Instructions  173 # In each of the following lines, there are three required fields:... https://perl5.git.perl.org/perl5.git/blob/HEAD:/t/op/sprintf.t#l173 https://perl5.git.perl.org/perl5.git/blob/HEAD:/t/op/sprintf.t#l481 
481 >%s< >sprintf('%%n%n %d', $n, $n)< >%n 2< >Slight sneakiness to te
+st %n<
 482 >%s< >$n="abc"; sprintf(' %n%s', substr($n,1,1), $n)< > a1c< >%n 
+w/magic<
 483 >%s< >no warnings; sprintf('%s%n', chr(256)x5, $n),$n< >5< >Unico
+de %n<
 484 >

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I imagine running the test file might show whats executed

In reply to Re: How to use sprintf %n formatting pattern by Anonymous Monk
in thread How to use sprintf %n formatting pattern by ibm1620

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