Ok.
On Windows, when you print "\n", there will be 2 characters, 0x0D 0x0A.
Verified using a binary editor.
When reading said text file under Windows, with standard text file I/O, Perl will remove the 0x0D (<CR>) character. That happens as part of the I/O layer and is invisible to the Perl user program.
I had thought that "\n" and "\n" had the same meaning whether write or read. It turns out that is NOT true.

Some Windows test code:

use warnings;
use strict;

open (my $file, '>', "testendings")
   or die "unable to open testendings for write! $!";
    
print $file "test\n"; 
close $file;
print "Binary file created: 74 65 73 74 0D 0A\n";

open (my $infile, '<', "testendings")
   or die "unable to open testendings for read! $!";   
   
my $in =<$infile>;
close $infile;

print "input as read by normal string IO: $in";
print "Length in bytes of input var as read is: ".length($in)," bytes\
+n";

$in =~ s/(.)/sprintf("%02X ",ord($1))/seg;
print "$in\n";
print "note: when using Perl text read, the 0x0D was deleted!\n";

__END__
Binary file created: 74 65 73 74 0D 0A  # 0D=>CR 0A=>LF
input as read by normal string IO: test
Length in bytes of input var as read is: 5 bytes
74 65 73 74 0A 
note: when using text read, the 0x0D was deleted!
[download]

My explanation of why "\r\n" doesn't work is functionally correct, but incorrect in the dirty details of Why?. On Windows this will print <CR><CR><LF>. When read back via text mode, only one <CR> will be deleted. The regex fails because there is still another <CR> there and "$" is looking for a 0x0A. Correct?

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