[Perl] system command timeout implementation

An example

This example is from perldoc

• Reference: $perldoc -f alarm

# eval returns the value of last expression evaluated
# if no timeout, return value will be 'alarm 0;', which return 0
# otherwise, the last statement executed is 'sub { die "alarm\n" };'
# since die within eval will stuff error message into $@, the $@ will contain "alarm"

eval {
    local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
    # after $timeout seconds, it will issue SIGALRM signal to 
    # this process unless it is cancelled by a new alarm (like 
    # 'alarm 0;' below)
    alarm $timeout;
    $nread = sysread SOCKET, $buffer, $size;
    alarm 0;
};

# if $@ is 0, no timeout happen, otherwise, $@ contains error message "alarm"
if ($@) {
    # propagate unexpected errors (if it is not timeout, die)
    die unless $@ eq "alarm\n";   
    # timed out
}
else {
    # didn’t
}