$ perl -Mstrict -Mutf8 -MBerkeleyDB -MEncode -MData::Dumper -le '
  unlink "xx.db";
  tie my %h, "BerkeleyDB::Btree", -Filename=>"xx.db", -Flags=>DB_CREATE;
  my $db=tied %h; $Data::Dumper::Useqq=1;
  $db->filter_fetch_key(
    sub {
      warn ">>fetch: ".Dumper($_);
      $_=decode("utf8", $_);
      warn "<<fetch: ".Dumper($_);
    });
  $db->filter_store_key(
    sub {
      warn ">>store: ".Dumper($_);
      $_=encode("utf8", $_);
      warn "<<store: ".Dumper($_);
    });
  $h{"ä"}=1;
  my @l=keys %h'
>>store: $VAR1 = "\x{e4}";
<<store: $VAR1 = "\303\244";
>>fetch: $VAR1 = "\303\244";
<<fetch: $VAR1 = "\x{e4}";

##</code><code>##

>>store_key: $VAR1 = "\x{e4}\x{fc}";
<<store_key: $VAR1 = "\303\244\303\274";
>>store_key: $VAR1 = "\x{e4}\x{e4}";
<<store_key: $VAR1 = "\303\244\303\244";

##</code><code>##

>>store_key: $VAR1 = "\x{e4}";
<<store_key: $VAR1 = "\303\244";
>>fetch_key: $VAR1 = "\x{e4}\x{e4}";
<<fetch_key: $VAR1 = "\x{fffd}\x{fffd}";

##</code><code>##

  my $check=qr/\A\Q$prefix\E(.)?/;

  $k=$prefix;
  if( ($rc=$cursor->c_get($k, $v, DB_SET_RANGE))==0 and $k=~$check ) {
    do {
      if( defined $1 ) {
        ...
      }
    } while( ($rc=$cursor->c_get($k, $v, DB_NEXT))==0 and $k=~$check );
  }