use Scalar::Util qw(readonly);;

my %hash = ( A => 1 );

while ( my ( $abc, $num ) = each %hash ) {
  print "$abc\n";
  print "$abc is readonly\n" if readonly($abc);
  my $bar = $abc;
  print "bar is readonly\n" if readonly($bar);
  foo($abc);
  print "$abc is still readonly\n" if readonly($abc);
}

sub foo {
  $_[0] .= 'FOO';
}

##</code><code>##

A
A is readonly
bar is readonly