Try this:

my @keys = qw(a b c d);
my $hash_ref; 
@{ $hash_ref }{@keys} = (1) x @keys;
print "$_\n" for keys %$hash_ref;
__END__
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In general, $h{x}{y} is shorthand for $h{x}->{y}, meaning that the content of $h{x} is a hash ref, just like $hash_ref above; once dereferenced, it can be used like any other hash.