Ok, Deparse shows

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;
[download]

So on windows "\n" becomes the bytes "\r\n" when you print, so 78 + 2 = 80, thats the lines

so seek to 1*80 from zero print "1" 78 times followed by "\r\n"

so seek to 2*80 from zero print "2" 78 times followed by "\r\n"

so seek to 3*80 from zero print "3" 78 times followed by "\r\n"

so seek to 4*80 from zero print "4" 78 times followed by "\r\n"

Then when reading the file the first 80 chars are null ("\0") then its 1111...222...333...4444

Thats Basic debugging checklist for you