You may wish to wrap your code in <code> tags next time. I'm not 100% sure what you're asking, but perhaps this code example will answer your question:

sub function {
   my $hashref = shift;

   $hashref->{three} = 3;
}

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

You can also have the same behavior like this:

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

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

Then there's this:

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

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