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sub foo{
    my ($hashref) = @_;
...
my %hash = (Gobs=>'many', of=>'loads', data=>'Huge');
...
foo( \%hash);

sub foo{
    my (%hash) = @_; #! Wrong!
...
}
...
foo( %hash ); #! Wrong!

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

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