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

##</code><code>##

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

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

##</code><code>##

%s =
  map
  { (split /_/, $_, 2)[-1] => $def{ $_ } }
  #  Useless use of grep.
  grep /^default_option_/, keys %def
  ;

##</code><code>##

%s =
  map
  { m/^ default_ (option_.+) $/x
    ? ( $1 => $def{ $_ } )
    : ()
  }
  keys %def ;