You can't "replace a key" but you can set a new one and delete the existing one
And you can do that in one line: 
$hash{$hash_tr_fr{$eng}} = delete $hash{$eng};

[download]
which has the added benefit to even work correctly when $hash_tr_fr{$eng} eq $eng. Doing the delete in a separate step would result in a loss of the entry.

In reply to Re^2: how can i replace a hash key with other hash value? by JavaFan
in thread how can i replace a hash key with other hash value? by satzbu

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