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sub function {
   my $hashref = shift;
...
my %hash = ( one => 1, two => 2 );
&function(\%hash);
print join(", ", keys %hash);   # one, two, three

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

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

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

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