Cheers all,

I recently stumbled over $subject -

for $foo ('a'..'c') {
    open FH,'|-', 'cat -';
    select FH;
    for $bar (1..rand 10) { write }
    $- = 0;
    close FH;
}
format FH_TOP = 
=@=
$foo
.
format FH =
 @
$bar
.
__END__
=a=
 1
 2
 3
 4
 5
 1
 2
 3
 4
 5
 1
 2
[download]

switching the lines with the $- assignment and close yields

for $foo ('a'..'c') {
    open FH,'|-', 'cat -';
    select FH;
    for $bar (1..rand 10) { write }
    close FH;
    $- = 0;
}
format FH_TOP = 
=@=
$foo
.
format FH =
 @
$bar
.
__END__
=a=
 1
 2
 3
 4
=b=
 1
 2
=c=
 1
 2
[download]

If the filehandle is pristine, $- is 0. But after the first close it is set to $= (page length), thus write skips format TOP.

In the source, in Perl_do_close() in doio.c (thanks mtve for pointing to it):

    if (not_implicit) {
        IoLINES(io) = 0;
        IoPAGE(io) = 0;
        IoLINES_LEFT(io) = IoPAGE_LEN(io);
    }
[download]

Why?

It's not documented either. Anybody knows the rationale for that? - I know, that's an old corner of perl. I just want to know.