Re: map2 {} grep2 {} ...

[ikegami]

You don't even use the key in

my %h_prefixed = map2 { ($_[0], "p".$_[1]) } %h;

You could do

my %h_prefixed = %h;
$_ = "p$_" for values %h_prefixed;

[BrowserUk]

He *did* use the keys -- to construct the new hash.

And you also used the keys, when you constructed the new hash.

But you also used the values at point -- and then again when you modified them.