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for (grep /^default_option_/, keys %def) {
   my (undef, $option) = split /_/, $_, 2;
   $s{$option} = $def{$_};
}

for ( grep /^default_option_/, keys %def ) {

   $s{ (split /_/, $_, 2)[-1] } = $def{ $_ };
}

%s =
  map
...
  #  Useless use of grep.
  grep /^default_option_/, keys %def
  ;

%s =
  map
...
    : ()
  }
  keys %def ;