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;