The following mundane piece of code does piped opens to 3 external processes that will each run for 3 seconds, and then times them out after 2, 3 & 4 seconds respectively. It gathers whatever output they produce before being timed out and (attempts) to return it to the caller.

UPDATE: Code and trace below updated to reflect andal's post below, and the information that error 1 below is fixed in 5.16. Error 2, the "modification of readonly value" remains; though in a slightly changed form as reflected by the new trace.

See the console trace after the __END__ token for the results::

#! perl -slw
use strict;
use threads stack_size => 4096;
use threads::shared;

sub timedCommand {
    my( $timeout, $cmd, @args ) = @_;

    my $pid :shared;
    my @input :shared;

    async {
#        local $_; ### LINE C ###
        $pid = open my $fh, "$cmd @args |" or die "$!, $^E";
        push @input, $_ while <$fh>;
    }->detach;
    sleep 1 until $pid;
    sleep 1 while kill 0, $pid and $timeout--;
    kill 3, $pid if $timeout;

    print "@input"; ### LINE A ###
    return @input;
}

my $cmd = q[ perl -E"$|++; sleep(1), say for 1 .. $ARGV[0]" ];

print timedCommand( 3, $cmd, $_ ) for 2, 3, 4; ### LINE B ###
__END__
C:\test>\perl64-16\bin\perl.exe timedCommand.pl
Thread 1 terminated abnormally: Modification of a read-only value atte
+mpted at timedCommand.pl line 15.


Thread 2 terminated abnormally: Modification of a read-only value atte
+mpted at timedCommand.pl line 15.
[download]

The two mysteries are:

1. Where does the contents of @input disappear to between LINE A and LINE B?

   Update: This part is fixed in 5.16.

2. Why, if I comment out LINE C, do I get:

   C:\test>timedCommand.pl
   Thread 1 terminated abnormally: Modification of a read-only value atte
   +mpted at C:\test\timedCommand.pl line 15.
   Thread 2 terminated abnormally: Modification of a read-only value atte
   +mpted at C:\test\timedCommand.pl line 15.
   Thread 3 terminated abnormally: Modification of a read-only value atte
   +mpted at C:\test\timedCommand.pl line 15.
   [download]

   UPDATE: This problem remains, though with slightly different output. Only two copies of teh error message are produced.

Perl bugs or bad code?