c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @ra = (1, 2, 3);
 my %hash = ( a => 'ok',  b => \@ra, );
 ;;
 S(\%hash);
 ;;
 dd \@ra;
 dd \%hash;
 ;;
 sub S {
   my $hashref = shift;
   my %hashcopy = %$hashref;
   ;;
   return do_something_with(\%hashcopy);
   }
 ;;
 sub do_something_with {
   my $hashref = shift;
   ;;
   $hashref->{b}[0] = 9;
   $hashref->{b}[1] = 9;
   $hashref->{b}[2] = 9;
   return 1;
   }
"
[9, 9, 9]

{ a => "ok", b => [9, 9, 9] }