use warnings;
use strict;
use Data::Dump qw(dump);


my %h = (
	1 => {
		1 => 'foo',
		2 => 'bar',
	},
	2 => [],
	3 => 'foo',
);

clear(\%h);

sub clear{
	my $ref = shift;
	%$ref->{1} = ();   # { 1 => undef, 2 => [], 3 => "foo" }
	%{$ref->{1}} = (); # { 1 => {}, 2 => [], 3 => "foo" }
}

print dump \%h;