From @LLFourn

1. by itself
#lib/noprecomp.pm
no precompilation;
my $_;

perl6 -Ilib -e 'need noprecomp;'

Use of uninitialized value %ENV of type Any in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things, if
needed. in any mop_up_and_check at src/Perl6/World.nqp​:401
Potential difficulties​:
  Redeclaration of symbol $_
...

2. from another module (dies painfully)
#lib/noprecomp2;
no precompilation;
need noprecomp;

perl6 -Ilib -e 'need noprecomp2;'

Use of uninitialized value %ENV of type Any in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things, if
needed. in any mop_up_and_check at src/Perl6/World.nqp​:401
===SORRY!===
This type cannot unbox to a native integer

Much of the problems comes from @​*MODULES not being used consistently​:

in pre curli code it's entries are assumed to be hashes​:
https://github.com/rakudo/rakudo/blob/nom/src/core/Exception.pm#L512

however in new code it's just an array of strings​:
https://github.com/rakudo/rakudo/blob/nom/src/core/CompUnitRepo.pm#L81

From @LLFourn

The "Use of uninitialized value %ENV of type Any in string context" is a
separate bug similar to​:

https://rt.perl.org/Public/Bug/Display.html?id=126447

It happens because with "no precompilation" the value of
ENV<RAKUDO_PRECOMP_DIST> disappears in a similar way to that RT.
Then when %*ENV is cloned the lack of a value there causes the error at​:

https://github.com/rakudo/rakudo/blob/nom/src/core/control.pm#L263

On Sat, Dec 12, 2015 at 2​:10 AM perl6 via RT <perl6-bugs-followup@​perl.org>
wrote​:

Greetings,

This message has been automatically generated in response to the
creation of a trouble ticket regarding​:
"[BUG] no precompilation with compile time warning dies painfully",
a summary of which appears below.

There is no need to reply to this message right now. Your ticket has been
assigned an ID of [perl #​126880].

Please include the string​:

     \[perl #​126880\]

in the subject line of all future correspondence about this issue. To do
so,
you may reply to this message.

                    Thank you,
                    perl6\-bugs\-followup@​perl\.org

-------------------------------------------------------------------------
1. by itself
#lib/noprecomp.pm
no precompilation;
my $_;

perl6 -Ilib -e 'need noprecomp;'

Use of uninitialized value %ENV of type Any in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things, if
needed. in any mop_up_and_check at src/Perl6/World.nqp​:401
Potential difficulties​:
Redeclaration of symbol $_
...

2. from another module (dies painfully)
#lib/noprecomp2;
no precompilation;
need noprecomp;

perl6 -Ilib -e 'need noprecomp2;'

Use of uninitialized value %ENV of type Any in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things, if
needed. in any mop_up_and_check at src/Perl6/World.nqp​:401
===SORRY!===
This type cannot unbox to a native integer

Much of the problems comes from @​*MODULES not being used consistently​:

in pre curli code it's entries are assumed to be hashes​:
https://github.com/rakudo/rakudo/blob/nom/src/core/Exception.pm#L512

however in new code it's just an array of strings​:
https://github.com/rakudo/rakudo/blob/nom/src/core/CompUnitRepo.pm#L81