For this reason, the filedescriptor is considered invalid from the PerlIO point of view

But it's not, or at least not completely invalid. You can still seek using the handle and print to the handle without problem. For example, adding

seek(STDOUT, -0, 2) or die $!;
print STDOUT "abc\n";
[download]

does indeed append "abc\n" to the file.

It's more like Perl remembers the handle's original mode and doesn't realize it can read from it now.

Update: I did a bit of Dumping and stracing of my own.

There's is no difference in the IO objects. I'm now with you leaning towards a PerlIO problem.

Seems that the "Bad file descriptor" message originates from Perl, not the system. Perl doesn't even attempt to read from STDOUT.

$ cat a.pl
use Devel::Peek;

open(SAVOUT, '>&STDOUT') or die $!;
close(STDOUT) if $ARGV[0];
open(STDOUT, '+>', "/tmp/stdout.log") or die $!;

Dump(*STDOUT{IO});

@argv = qw(/bin/echo hello world);
system(@argv);

print SAVOUT "before=", tell(STDOUT), "\n";
seek(STDOUT, 0, 0) or die $!;
print SAVOUT "after=", tell(STDOUT), "\n";
while (1) {
  my $rv = read STDOUT, $_, 8192;
  die $! if !defined($rv);
  last unless $_;
  print SAVOUT "stdout=", $_;
}
print SAVOUT "at end=", tell(STDOUT), "\n";
close STDOUT;

$ diff -u <(strace perl a.pl 0 2>&1) <(strace perl a.pl 1 2>&1) | less
...
 lseek(1, 0, SEEK_SET)                   = 0
 lseek(1, 0, SEEK_CUR)                   = 0
-[ code to read locale-dependent version of error message]
-write(2, "Bad file descriptor at a.pl line"..., 37Bad file descriptor
+ at a.pl line 17.
-) = 37
+read(1, "hello world\n", 4096)          = 12
+read(1, "", 4096)                       = 0
+close(1)                                = 0
-write(3, "before=0\nafter=0\n", 17before=0
+write(3, "before=0\nafter=0\nstdout=hello wo"..., 46before=0
 after=0
-)     = 17
+stdout=hello world
+at end=12
+) = 46
 close(3)                                = 0
-exit_group(9)                           = ?
-Process 4028 detached
+exit_group(0)                           = ?
+Process 4032 detached
[download]

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