Yes, but is it a useful copy? The question was about an extra reference-dereference pair that makes a copy of a copy. What do you get for that work?

Just about nothing. There is no reason that I can see to prefer:

sub foo {
  my @foo = @{[@{$_[0]}]};
  # etc
}
[download]

over

sub foo {
  my @foo = @{$_[0]};
  # etc
}
[download]

In fact try this:

my $bar = ["hello", "world"];
foo($bar);
print "$bar->[0]\n";

sub foo {
  # Make my copy.
  my @foo = @{$_[0]};
  # Try to mess up the first element.
  $foo[0] =~ s/h/H/;
}
[download]

See? You get a private copy already.