In place, you could do something like this:

for (@HoL{keys %HoL}) {
  $_ = {map {$_, 1} @$_};
}
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(You can use values in place of the slice in 5.6, but I'm not there yet.)

If you really need a copy, that's a little odder, like this:

%HoH = map { $_ => {map { $_, 1 } @{$HoL{$_}}} } keys %HoL;
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Untested, but I think I got my indirections right.

-- Randal L. Schwartz, Perl hacker