sub foo{
    my ($hashref) = @_;
    for my $key (keys %{$hashref}) {
        do stuff;
    }
}

my %hash = (Gobs=>'many', of=>'loads', data=>'Huge');
...
foo( \%hash);

##</code><code>##

sub foo{
    my (%hash) = @_; #! Wrong!
    for my $key (keys %hash) {
        do stuff
    }
}
...
foo( %hash ); #! Wrong!

##</code><code>##

exists $b{$k} or delete $a{$k} while ($k,$v) = each %a;

##</code><code>##

exists $b{$k} and $c{$k}=$v while ($k,$v) = each %a