my %hash = ( ... );
map { $hash{$_} = ($hash{$_} * 2) } keys %hash;

##</code><code>##

my %hash = ( ... );
my %new_hash = map {  $_ => ($hash{$_} * 2) } keys %hash;

##</code><code>##

sub length {
    my ($h, @t) = @_;
    return 0 unless defined $h;
    return 1 + length(@t);
}