Thanx for the pointer, almut. That dup & perlio scrap is interesting.

But there must be more to it than that, as I don't see any special treatment for STDOUT in the perlio.c scrap (neither for the numeric FD's 0 to 2):

I was playing with the scrap below in the meantime.

I dupped SAVOUT on STDERR instead / simplifying system to printing / using autoflush / opening STDOUT myself to /dev/tty first: no change.

This however is interesting:

Changing the name of the handle STDOUT <=> ANYTHINGeLSE manages to act as a toggle for the problem. Furthermore, w/o close, the tell on the STDOUT file pointer at begin prints 19 in the example below (might be due to the handle earlier being a tty, and something didn't quite catch the change to a plain file w/o explicit close?). Any other handle name prints 5 regardless of close or no close.

So it looks like we have some hard-coded STDOUT-related magic somewhere in the guts of PERLIO or even lower, with probably STDIN/ERR offering similar peculiarities.

use IO::Handle;

open(SAVOUT, '>&STDERR') or die $!;
#open(SAVOUT, '>&STDOUT') or die $!;

# play with these:
# s/STDOUT/STDOUT/; s/STDOUT/STDOUT/

close STDOUT;
open(STDOUT, '>>', "/dev/tty");
STDOUT->autoflush;
print STDOUT "a STDOUT test\n";

# play with this:
# close STDOUT;

open(STDOUT, '+>', "/tmp/STDOUT.log") or die $!;
STDOUT->autoflush;
print STDOUT "test\n";
#system("echo hello world");

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 "read: $!\n" if !defined($rv);

  last unless $_;
  print SAVOUT "STDOUT=", $_;
}
print SAVOUT "at end=", tell(STDOUT), "\n";
close STDOUT;
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Given that too much in Perl, esp wrt <> and stdio is magic, it's probably a good idea to say strictly outside any possibly dusty corner whose smell is faintly related to something magic. Which in this case might just be the idea of reusing a special handle, and worse, reading from it.

Still wondering (& vowing to step even more cautiously anywhere near STDIO magic),

My thanx to almut & ikegami for the work below!

less confused now (& busy scribbling away two new-to-me debugging tips along with a link to their demonstration here)
Peter