I expected this to just turn off all handlers. Why do i need to write local $SIG{__DIE__};? Can anybody explain this?

Localizing a hash just creates an empty alias, with no key whatsoever. With local $SIG{__DIE__} you explicitly set the __DIE__ handler to undef, so for this context, you get back the default behavior of die, and die is done, signal processed. If you just localize %SIG, the __DIE__ handler of the surrounding context is still in place, since the __DIE__ signal isn't handled by the current localized %SIG (there is no __DIE__ key in %SIG) - so it is passed up and hits the handler installed before the localization. Does that make sense to you?

Update: the question from the previous paragraph just indicates that the explanation is poor, i.e. uncomplete. Further then:

Localizing a hash just creates an empty alias, with no key whatsoever. The processing of a signal via a custom handler is dependent on the presence of the signal name as key in the hash %SIG.

If you say local $SIG{__DIE__}, you just alias the value of the slot __DIE__ of the %SIG currently in place. If you localize the entire %SIG without further ado, this has no effect whatsoever, since it has no keys. Aliasing doesn't create keys! So, if you write something like 

$SIG{__DIE__}= sub {print "fubar!\n"};
eval {
    local %SIG;
    local $SIG{__DIE__};
    $a/$b
};
print $@;

[download]

you will see

fubar!
Illegal division by zero at - line 5.

[download]

since the subsequent aliasing of an undefined value in the already localized hash %SIG doesn't create the key __DIE__ in %SIG.

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

In reply to Re: Localizing %SIG by shmem
in thread Localizing %SIG by Darkwing

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