$other_hash{key} = '';

$hash{other} = \%other_hash; 
$hash{other}->{key}             = '1'; # ok
$tmp = $hash{other}; $$tmp{key} = '2'; # use it like this
# $$hash{other}{key}            = '2'; # but not like this
${$hash{other}}{key}            = '3'; # this is ok
$hash{other}{key}               = '3'; # this too, -> unneeded after {}