Is this a mistake in this module? Or have I somehow messed up and perl on windows? Is perl meant to treat '\n' in regex as '\r\n' when running on windows?

As others have pointed out, Perl normally uses the :crlf PerlIO layer on Windows. However, if you look at how _parse_hosts gets called, it's using AnyEvent::IO's aio_load, which does open $fh, "<:raw:perlio", or AFAICT its XS equivalent when the IO::AIO backend is used. So while you could hack it by changing the file to use LF instead of CRLF - I have no idea whether other Windows libraries will like that, though! - or by monkey-patching _parse_hosts*, I'm not sure you'll be able to change this behavior without getting in touch with the module's author.

* Just for fun, this ugly hack should work:

{
  my $oldsub = \&AnyEvent::Socket::_parse_hosts;
  no warnings 'redefine';
  *AnyEvent::Socket::_parse_hosts = sub ($) {
    (my $in = shift) =~ s/\r\n/\n/g; $oldsub->($in) };
}
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Update: Looking at the code, another possible workaround is to point the environment variable PERL_ANYEVENT_HOSTS at a hosts file that has only LF line endings.

Thanks for the great answer, I have certainly learned more about IO layers now. Your hack idea is better than my redefine hack - I had not thought of calling the old sub, I just redefined it with a fixed version.

If you open it with :crlf (which is the default on MSWin)

If you're running perl "on Cygwin on MSWin" it looks to me that :crlf is not the default.
I therefore deduce that you regard running perl "on Cygwin on MSWin" as not running perl "on MSWin".

(Just trying to clear up exactly what it means to be "on MSWin".)

Cheers,
Rob

Yeah, by "running perl on MSWin" I mean

$^O eq 'MSWin32'
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map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

I encountered the issue running standard strawberry perl 64bit. Modifying hosts file wasn't really an option for me because the idea was the program is being pp'd into an exe and run by various users on their own systems.

On Windows, Perl automatically converts CRLF to LF on read, and LF to CRLF on write (by default).

But you might be using a perl built as part of unix emulation environment like Cygwin and I think MSYS/MSYS2. Such a Perl wouldn't perform such translations, even though it could be launched from a Windows program (e.g. the Windows shell).

Possible solutions:

• You could install a native build of Perl (Strawberry Perl or ActivePerl).
• You could convert the file's line endings (e.g. using Perl or dos2unix).
• You could monkeypatch the module. (See haukex's answer.)
• You could patch the module.