perl -MData::Rmap=rmap_hash -le 'rmap_hash {print keys %{$_->{a}} } ({a=>{1,2}})'

##</code><code>##

push @return, $self->_rmap($_) for values %$_;

##</code><code>##

# added exists check
perl -MData::Rmap=rmap_hash -le 'rmap_hash {print keys %{$_->{a}} if exists $_->{a} } ({a=>{1,2}})'
1