"If there was no error, $@ is guaranteed to be a null string."

Yeah, but it's useless, because the time frame where $@ is guaranteed to be a null string is less than the width of a semi-colon:

sub DESTROY {$@ = "Hi"}
eval {bless []};
say $@;
__END__
Hi!
[download]

No error, but between the finishing of the eval, and the first statement after the eval, the return value of eval is garbage collected, setting $@.