# Abuse the symbol table.  This does not work under strict.
sub a1 {
  local *c = shift;
  $c{a} = 'a';
}

# Copy the hash by force.  Very inefficient.
sub a2 {
  my $ref = shift;
  my %c = %$ref;
  $c{a} = 'a';
  %$ref = %c;
}

##</code><code>##

# "Arrow" notation.
sub a3 {
  my $ref = shift;
  $ref->{a} = 'a';
}

# Or compactly
sub a4 {
  (shift)->{a} = 'a';
}