sub function {
   my $hashref = shift;

   $hashref->{three} = 3;
}

my %hash = ( one => 1, two => 2 );
&function(\%hash);
print join(", ", keys %hash);   # one, two, three

##</code><code>##

sub function (%) {
   my %hashref = shift;
   ...
}

&function(%hash);       # passes %hash as a ref

##</code><code>##

sub function {
   my %hash = %{shift};   # de-references

   $hash{three} = 3;      # Does not affect the real %hash!
}