require v5.16;
use Test::More qw(no_plan);

for(51..66)
 {my $s = sprintf("%b", 1 << $_ -1);
  is length($s), $_, "1 << $_ -1";
 }
__END__
ok 1 - (1 << 51) -1
ok 2 - (1 << 52) -1
ok 3 - (1 << 53) -1
ok 4 - (1 << 54) -1
ok 5 - (1 << 55) -1
ok 6 - (1 << 56) -1
ok 7 - (1 << 57) -1
ok 8 - (1 << 58) -1
ok 9 - (1 << 59) -1
ok 10 - (1 << 60) -1
ok 11 - (1 << 61) -1
ok 12 - (1 << 62) -1
ok 13 - (1 << 63) -1
ok 14 - (1 << 64) -1
not ok 15 - (1 << 65) -1
#   Failed test '(1 << 65) -1'
#   at - line 6.
#          got: '64'
#     expected: '65'
not ok 16 - (1 << 66) -1
#   Failed test '(1 << 66) -1'
#   at - line 6.
#          got: '64'
#     expected: '66'
1..16
# Looks like you failed 2 tests of 16.