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my %hash;
@hash{ 3,4 } = qw( perl hacker );
print join ",", keys %hash;      # prints "3,4"

my %hash;
$hash{ 3,4 } = qw( perl hacker );
print join ",", keys %hash;      # ????

my %hash;
$hash{ (3,4) } = qw( perl hacker );
print join ",", keys %hash;      # ????

my %hash;
$hash{ [3,4] } = qw( perl hacker );
print join ",", keys %hash;      # ????

my %hash;
$hash{ 3,4 } = qw( perl hacker );
print $hash{ 3,4 }, "\n";

my %hash;
for my $i (0..5) {
...
  }
}
foreach ( keys %hash ) { print "$_ : ", $hash{ $_ }, "\n"; }