Hashes can be passed either by value:

  mysub(%hash);

  sub mysub {
    my (%params) = @_;
  }
[download]

or by reference:

  mysub(\%hash);

  sub mysub {
    my $params = shift;
    my %paramhash = %$params;
  }
[download]

Each approach has it's advantages and disadvantages. Pass-by-value offers a simpler syntax, while pass by reference is faster and more flexible, allowing you to easily intermingle the hash with other parameters.