[Perl] signal handler

1. Use %SIG hash to trap control signal

You can set the values of the %SIG hash to be the functions you want to
handle the signal. After perl catches the signal, it looks in %SIG for a
key with the same name as the signal, then calls the subroutine value for
that key. The following is signal handler for INT signal [From perldoc -q SIG]

# as an anonymous subroutine
$SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };

# or a reference to a function
$SIG{INT} = \&ouch;

# or the name of the function as a string
$SIG{INT} = "ouch";

sub ouch {
    die "Caught a signal: $!";
}

We can define handlers for other signals as well.

$SIG{TERM} = sub { print STDERR, "Going to terminate."; };
$SIG{__DIE__} = sub { die "Going to die:".shift; };
$SIG{__WARN__} = sub { die "Going to die:".shift; };

The key of %SIG hash can be INT, ‘INT’, and __INT__.

signal handler also can be defined with eval, such as

eval {
       local $SIG{ALRM} = sub { die "timeout\n" };
};

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