#!/usr/bin/perl
use strict;
use warnings;

use Inline C => <<'END_C';

void 
readonly(SV *sv) {
    SvREADONLY_on(sv);
}

END_C

my $foo = "foo";
readonly($foo);

# This dies with: 
# "Modification of a read-only value attempted at cat_readonly.plx line 16."
$foo .= "bar";

# unreachable:
print "$foo\n";