my %months = (
  'Jan' => 31,
  'Feb' => 28,
  'Mar' => 31);

# this kills the whole hash
undef %months;

# this does NOT work
undef $months{'Feb'};

# this does not work either
$months{'Feb'} = undef;

# this deletes entry 'Feb':
delete $months{'Feb'};

# this is equivalent to deleting the whole hash:
foreach $key (keys %months) {
  delete $months{$key};
}

# this will also delete all keys:
delete @months{keys %months}

# another way to empty the hash
%months = ();