use strict;
use warnings;
require Data::Dumper;

my $foo = {  # blessed
  one => 1,
  two => 2,
  six => 6,
};
bless $foo, q{Number::3ch};

my $bar = {  # not blessed
  one => 1,
  two => 2,
  six => 6,
};

sub foo {    # pass by ref
  my $ref = shift;
  $ref->{one} = q{One};
  $ref = {}; # why won't this replace the underlying hash?
  return $ref;
}

# try on blessed reference
print Data::Dumper::Dumper($foo);
foo $foo;
print Data::Dumper::Dumper($foo);

# try on unblessed reference
print Data::Dumper::Dumper($bar);
foo $bar;
print Data::Dumper::Dumper($bar);