why can I open undef?

Question

The following does not die:

open my $in, '<', undef or
    die q{couldn't open undef};
print <$in>;

Neither does this:

open my $in, '>', undef or
    die q{couldn't open undef};
print $in 'hello';

I don't understand why neither of these die. How could opening undef possibly be successful? The reason I found this was that a guy I work with had done this:

open my $in, '>', $ARGV[0] or die q{couldn't open $ARGV[0]};

He thought that this would kill the script if no arguments were passed in (I know this isn't the cleanest way to do that but I didn't think it wouldn't work).

I'm using Strawberry 5.16.1.

Answer 1

See perldoc -f open:

As a special case the three-argument form with a read/write mode and the third argument being undef:

open(my $tmp, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file.