Confused about close STDIN

Looking at and old bug report and getting a bit confused about the  
behaviour after closing STDIN...


First one:

#!/usr/bin/perl -l

use strict;
use warnings;

close STDIN;

open my $foo, "<", $0;
print fileno($foo);

while (<STDIN>) {
   chomp;
   print "<STDIN>: $_";
}

while (<>) {
   chomp;
   print "<> ($ARGV): $_";
}
__END__

Output:

readline() on closed filehandle STDIN at c.pl line 11.
<> (-): #!/usr/bin/perl -l
<> (-):
<> (-): use strict;
...

perlop says:
'The null filehandle <> is special: it can be used to emulate the  
behavior of sed and awk. Input from <> comes either from standard  
input, or from each file listed on the command line. Here's how it  
works: the first time <> is evaluated, the @ARGV array is checked, and  
if it is empty, $ARGV[0] is set to "-", which when opened gives you  
standard input.'

Which might be confusing... since it doesn't use STDIN but it uses fd  
0 directly (which normally is STDIN).



Second: (reduced from a bug report)

#!/usr/bin/perl -l

use strict;
use warnings;

close STDIN;
require IPC::Open2;
IPC::Open2->import;

open2(my $child_out, my $child_in, 'cat');
print "fileno(\$child_out): " . fileno($child_out);
print "fileno(\$child_in): " . fileno($child_in);
print $child_in "abc def";
close $child_in;

while (<$child_out>) {
   chomp;
   print "<\$child_out>: $_";
}
__END__
Output:

fileno($child_out): 5
fileno($child_in): 4

(in this case there is no output

Without the close STDIN:

#!/usr/bin/perl -l

use strict;
use warnings;

require IPC::Open2;
IPC::Open2->import;

open2(my $child_out, my $child_in, 'cat');
print "fileno(\$child_out): " . fileno($child_out);
print "fileno(\$child_in): " . fileno($child_in);
print $child_in "abc def";
close $child_in;

while (<$child_out>) {
   chomp;
   print "<\$child_out>: $_";
}
__END__
Output:

fileno($child_out): 5
fileno($child_in): 4
<$child_out>: abc def


Third one: adding an open after the close

#!/usr/bin/perl -l

use strict;
use warnings;

close STDIN;
open my $foo, "<", $0;

require IPC::Open2;
IPC::Open2->import;

open2(my $child_out, my $child_in, 'cat');
print "fileno(\$child_out): " . fileno($child_out);
print "fileno(\$child_in):  " . fileno($child_in);
print $child_in "abc def";
close $child_in;


while (<$child_out>) {
   chomp;
   print "<\$child_out>: $_";
}
__END__

Output (1):
fileno($child_out): 5
fileno($child_in):  4

Output (2):
fileno($child_out): 5
fileno($child_in):  4
<$child_out>: #!/usr/bin/perl -l
<$child_out>:
<$child_out>: use strict;
...

In this code the output changes between each run...
Sometimes I get output (1), sometimes output (2). (without changing  
the code obviously)


Should this behaviour be better documented (for example in perldoc -f close)?
Or is this a bug?


Kind regards,

Bram

• Confused about close STDIN by Bram
• Re: Confused about close STDIN by Mark Mielke