Of course you can pass by value & make changes sans changes to original reference via dereference ...

#!/perl

use warnings; use strict; use 5.010;

my $got = throw_ref();
my %hash = change_hash( %{ $got } );

say 'original hash now ...';
show_hash( %{ $got  } );

exit;

sub change_hash
{
  my ( %h ) = @_;

  say 'before modification ...';
  show_hash( %h );

  $h{'p'} += 10;
  $h{'q'} = 3;

  say 'after modification ...';
  show_hash( %h );

  return %h;
}

sub show_hash
{
  my ( %h ) = @_;
  printf "%s : %s\n" , $_ , $h{ $_ } for sort keys %h;
  return;
}

sub throw_ref
{
  return { 'p' => -2 } ;
}

__END__
before modification ...
p : -2
after modification ...
p : 8
q : 3
original hash now ...
p : -2
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... But if the hash reference is big and/or tied, then dereference could be expensive and/or undesired.

Sorry, this makes no sense. If the sub returns a reference to a hash, just dereference it and pass the hash. You can easily convert between scalar and reference. I suggest looking at this.

I don't understand why you can't pass by value, but no matter, you can just dereference inside the sub.

sub mysub{
    my $h = shift;
    my %dereferenced_hash = %$h;
    delete $dereferenced_hash{'b'};
}
[download]