By reading docs again and again I found my mistake. The $_ variable must be modified instead of returning value, because the return code from the filter is ignored. Thank you

The correct filtering function is

(tied %data)->filter_fetch_key( sub { $_ = decode('utf8', $_); } );

In reply to Re:^4 DBM modules and unicode keys by ph0enix
in thread DBM modules and unicode keys by ph0enix

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