sub some_meth {
  my($self, $source) = @_;
  my %target = (
    key1 => 'default_1',
    key2 => 'default_2',
    keyN => 'default_N',
    %$source,
  );
  # do somethings with %target
}

##</code><code>##

my %target = (
  # initialization
);
%target{keys %$source} = values %$source
  if defined $source && ref $source eq 'HASH';