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

[download]

... But if the hash reference is big and/or tied, then dereference could be expensive and/or undesired.

In reply to Re^3: Global variable unexpectedly modified when passed by reference by parv
in thread Global variable unexpectedly modified when passed by reference by fshrewsb

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