BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use threads;
use strict;
die $! unless open my $fh, '>', 'junk.dat';
$_->join foreach (map {async sub {
    seek $fh, $_ * 80, 0;
    print $fh $_ x 78;
}
;} 1..4);
close $fh;
die $! unless open $fh, '<', 'junk.dat';
print $_ while defined($_ = <$fh>);
close $fh;