Firstly, thanks for the reply. So did I understood this right, that in case that the signal_num isn't equal to 0 coredump is having a value too and the child died unexpected?

But I'm still asking myself, why I have to check this on the bitwise-operand way with if ($? & 127)?

Here some sample code:

my $pid = fork( );
        if ( defined $pid && $pid == 0 ) 
        {
            # child
            system( $_[0]->arg );
            if ($? == -1) 
            {
                return "failed to execute: $!\n";
            } 
            elsif ($? & 127) 
            {
                return "child died with signal %d, %s coredump\n", ($?
+ & 127),  ($? & 128) ? 'with' : 'without';
            } 
            else
            {
                return "child exited with value " . $? >> 8;
            }
        }
        else
        {
            # parent
        }
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