Wise monks,
I seek your wisdom to enlighten me in this matter:

In short terms:
Why does the close fail, when I try to close the filehandle
before the eof is reached?

my $cmd = '/usr/bin/strings /etc/passwd';
open my $handle, "$cmd |" or die "open: $cmd: $!\n";

# read at least one line
while ( <$handle> ) {
    last;
}

close $handle or die "$cmd; exit code($?): $!\n";

[download]

In longer terms, I tested several external commands and
tested each 100 times:

#!/usr/bin/perl
# vi:ts=4 sw=4 et:
use strict;
use warnings;

# these are available commands
my %cmd = (
    'cat'     => '/bin/cat',
    'less'    => '/usr/bin/less',
    'more'    => '/bin/more',
    'strings' => '/usr/bin/strings',

);

my $textfile = '/etc/passwd';


for my $k ( keys %cmd ) {
    my $success = 0;
    for ( 1 .. 100 ) {
        $success += open_close_pipe( $cmd{$k} );
    }

    print "$cmd{$k}: $success/100 successful!", $/;
}

sub open_close_pipe {

    my $ext_cmd = shift;
    my $cmd     = "$ext_cmd $textfile";

    open my $handle, "$cmd |" or die "open: $cmd: $!\n";

    # read at least one line
    while ( <$handle> ) {
        last;
    }

    close $handle 
        or do {
            #warn "exitcode($?): $cmd: $!\n"; # to see exit code ...
            return 0;
        };

    return 1;
}

__END__

[download]

This was my result:

$ perl read2.pl 
/bin/cat: 100/100 successful!
/usr/bin/less: 38/100 successful!
/bin/more: 31/100 successful!
/usr/bin/strings: 40/100 successful!

[download]

And the results vary (with the same code):

$ perl read2.pl 
/bin/cat: 100/100 successful!
/usr/bin/less: 16/100 successful!
/bin/more: 26/100 successful!
/usr/bin/strings: 32/100 successful!

[download]

When I close the handle directly after opening (comment out the while-loop), the result is like this:

$ perl read2.pl 
/bin/cat: 35/100 successful!
/usr/bin/less: 0/100 successful!
/bin/more: 41/100 successful!
/usr/bin/strings: 37/100 successful!

[download]

When I read from the handle till nothing's left (comment out the last inside the while-loop), it looks fine:

$ perl read2.pl 
/bin/cat: 100/100 successful!
/usr/bin/less: 100/100 successful!
/bin/more: 100/100 successful!
/usr/bin/strings: 100/100 successful!

[download]

Can you please enlighten me and tell me, why the close fails in most cases, when the handle hasn't reached eof?

I tested upon a Linux box with Perl v5.8.8 and Perl v.5.10.0.

edit: "longer terms" included into readmore

In reply to close fails(?) occasionally when closing handle of a reading pipe? by linuxer

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