Further to the suggestions of SilasTheMonk and thargas, an example. Note that numeric context must now be supplied explicitly. Also, I have no idea what this does to execution speed. Caveat Programmor.

c:\@Work\Perl>perl -wMstrict -le
"use Scalar::Util qw(dualvar);
 use Readonly;
 Readonly my $foo => dualvar 42, 'FOO';
 print qq{foo ($foo) = }, 0+$foo;
 $foo = '11';
 print qq{after ro mod};
"
foo (FOO) = 42
Modification of a read-only value attempted at -e line 1
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(Read-only modification also fails with numeric assignment.)