Judging by the fact that his DIRFILESEP was originally set to a single backslash, ExtUtils::MM_Win32's is_make_type() method doesn't seem to identify his make variant as either nmake or dmake, so that's where I would start digging...

(AFAICT, the implementation of the sub init_DIRFILESEP hasn't changed recently — at least all versions of EU::MM_W32 which shipped with various Perls going back to 5.8.8 do look the same in this respect.)

Here is that sub.

I can probably read perlre to figure out what ?: does, but a search of perlop for !! gives me nothing useful. Is that really two Boolean NOTs side-by-side? Can't be. So what is that operator? In any case, I'm guessing the real answer lies in the make method, and I'll pretend this sub makes sense even though I don't understand it now.

sub is_make_type {
    my($self, $type) = @_;
    return !! ($self->make =~ /\b$type(?:\.exe)?$/);
}
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I found the make method defined over in ExtUtils::MM_Any, which is required by MM_Win32.pm on line 27.

sub make {
    my $self = shift;

    my $make = lc $self->{MAKE};

    # Truncate anything like foomake6 to just foomake.
    $make =~ s/^(\w+make).*/$1/;

    # Turn gnumake into gmake.
    $make =~ s/^gnu/g/;

    return $make;
}
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So self->{MAKE} is some kind of OOesque hash key lookup, right? Sorry, but I don't know Perl OO very well.

But where does that key get set? Since that is an access to private data in this class (I think), doesn't that particular MAKE hash key value have to get set within this package file? But I can't find it in MM_Any.pm. Well, I find plenty of "MAKE" strings being used to demarcate here-files, but that isn't helping my search.

Any ideas?

Any ideas?

Stop digging before you've ruled out the most simple things

Step 1, step out of cpan, cpan>look Module and run perl Makefile.PL yourself, and confirm

perl -V:
<P>Confirm all paths, eg <C>perl -V:make -V:perlpath -e " die $^X
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Look inside Makefile for path to perl.exe

But where does that key get set?

There's init_MAKE(), which sets it (if it's not alredy set at this point) from the environment, or else from $Config{make} (which should be what you get via perl -V:make )

sub init_MAKE {
    my $self = shift;

    $self->{MAKE} ||= $ENV{MAKE} || $Config{make};
}
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A couple of well-placed prints can answer many questions, e.g. allow you to check whether the value is what you'd expect, or something funky

sub init_MAKE {
    my $self = shift;

    print STDERR ">>> before: '$self->{MAKE}'\n";
    $self->{MAKE} ||= $ENV{MAKE} || $Config{make};
    print STDERR ">>> after:  '$self->{MAKE}'\n";
}
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(and if you don't get any print output at all, you'll at least know you're executing something else entirely...)

P.S. For a discussion of !!, see Perl Idioms Explained - !!expr and Secret Perl Operators: the boolean list squash operator, x!! (and probably others) — if generally interested, it's not really relevant to the problem at hand.

My other posts in this thread show that I am now starting all over again with a virgin installation, but have the same problem.

I have not installed a new version of ExtUtils::MakeMaker. It looks like the version I have is 6.56.

C:\strawberry\perl\lib\ExtUtils
> find "our $VERSION" MakeMaker.pm

---------- MAKEMAKER.PM
our $VERSION = '6.56';
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That subroutine in MM_Win32.pm is identical to the one you've shown (but I added line numbers).

127 sub init_DIRFILESEP {
128     my($self) = shift;
129 
130     # The ^ makes sure its not interpreted as an escape in nmake
131     $self->{DIRFILESEP} = $self->is_make_type('nmake') ? '^\\' :
132                           $self->is_make_type('dmake') ? '\\\\'
133                                                        : '\\';
134 }
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And my Perl make configvar seems to point at dmake.

C:\strawberry\cpan\build
> perl -V:make
make='dmake';
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