Many months later I now know how eval{} can fail and yet not set $@. It happens if a DESTROY method called during the stack-unwind also does an eval. 
use AtExit;

eval {
    my $q = AtExit->new(sub{eval{}});
    die "Oops\n";
};

print $@; # Prints nothing

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In reply to Re^2: eval {require module};? by nobull
in thread eval {require module};? by Anonymous Monk

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