%h = map { $_ => $h{$_} } @to_keep;
is the same as
%h = @h{@to_keep};

Update: Oops, no! It would actually be equivalent to the more complicated: 

my %temp;
@temp{@to_keep} = @h{@to_keep};
%h = %temp;

[download]

In reply to Re^2: Hash assignments using map by ikegami
in thread Hash assignments using map by njcodewarrior

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