my %hash = ( all => 'old' );       # simple hash value
foo(\%hash);                       # pass reference to hash

sub foo {                          # recieve reference as $_[0]
  my $ref = shift;                 # copy reference to $ref (and throw away $_[0])
  $ref->{ all } = 'new';           # overwrite the value to the key 'all' 
                                   # in the hash pointed to by $ref (which is %hash)
}