my crude solution is to use string eval ...

What also seems to work is

Readonly my $_debug_ops => $DBG_RAND+0 | $DBG_KEYS+0 | $DBG_INFO+0;
[download]

but I'm not sure if it's the "+0 = force numeric" aspect or the associated pre-evaluation of the variables that's making this work...  (of course, this workaround isn't very satisfying either).

Anyhow, it's probably best to just stay away from using Readonly... (I had also already been bitten by other subtle weirdnesses).