use Scalar::Readonly qw/ readonly_on /;
use Devel::Peek qw/ Dump /;
my $f=66;
readonly_on($f);
Dump($f);
__END__
SV = IV(0x99ba38) at 0x99ba3c
  REFCNT = 1
  FLAGS = (PADMY,IOK,READONLY,pIOK)
  IV = 66