What is a sensible value for PERL_DL_NONLAZY when it comes to Catalyst development?

It should have no effect.

What version of List::MoreUtils do you have? In 0.26, the line in question is

local $ENV{PERL_DL_NONLAZY} = $ENV{PERL_DL_NONLAZY} ? 0 : $ENV{PERL_DL
+_NONLAZY};
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There no way that should warn. I don't see how an assignment would ever care about undef for either side, even taking into consideration that %ENV is magical. But I guess it's possible unless the error is being misreported. Then again, I don't see any source of warning in the source for scalar assignment in the latest Perl version.

Maybe something is messing with %ENV? Maybe you're facing a bug in (a rather old version of) Perl? Either way, the problem is not related to the value of PERL_DL_NONLAZY.