I ran your command, and it looks okay to to me.

C:\strawberry\cpan\build\OLE-Storage_Lite-0.19-pT5XuU
> perl -V:make -V:perlpath -e " die $^X"
make='dmake';
perlpath='C:\strawberry\perl\bin\perl.exe';
C:\strawberry\perl\bin\perl.exe at -e line 1.
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After the perl Makefile.PL command, I look in the Makefile and see:

C:\strawberry\cpan\build\OLE-Storage_Lite-0.19-pT5XuU
> find "perl.exe" Makefile

---------- MAKEFILE
PERL = C:\strawberry\perl\bin\perl.exe
FULLPERL = C:\strawberry\perl\bin\perl.exe
          perl.exe so_locations \
FULLPERL      = C:\strawberry\perl\bin\perl.exe
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Any more suggestions?

Any more suggestions?

In my version of Strawberry, when I build IO-React-1.03, there's no problem.
When I look at the generated Makefile I see:

DIRFILESEP = \\
DFSEP = $(DIRFILESEP)
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Next I open up lib/ExtUtils/MM_Win32.pm and I find in it:

sub init_DIRFILESEP {
    my($self) = shift;

    # The ^ makes sure its not interpreted as an escape in nmake
    $self->{DIRFILESEP} = $self->is_make_type('nmake') ? '^\\' :
                          $self->is_make_type('dmake') ? '\\\\'
                                                       : '\\';
}
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So I change the dmake setting (from 4 backslashes to 2) so that the sub now reads:

sub init_DIRFILESEP {
    my($self) = shift;

    # The ^ makes sure its not interpreted as an escape in nmake
    $self->{DIRFILESEP} = $self->is_make_type('nmake') ? '^\\' :
                          $self->is_make_type('dmake') ? '\\'
                                                       : '\\';
}
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Now ... have another go at building IO-React-1.03 (having first run 'dmake realclean'):

C:\sisyphusion\IO-React-1.03>perl makefile.PL
Checking if your kit is complete...
Looks good
Writing Makefile for IO::React
Writing MYMETA.yml and MYMETA.json

C:\sisyphusion\IO-React-1.03>dmake
dmake:  Error: -- `C:\_32\strawberry514\perl\libConfig.pm' not found, 
+and can't be made
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Sure enough, when I look at the generated Makefile, I now find it's the same as yours wrt DIRFILESEP:

DIRFILESEP = \
DFSEP = $(DIRFILESEP)
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Your generated Makefile is wrong ... plain and simple ... you need to remedy whatever it is that keeps generating this fucked Makefile.

In the above MM_Win32.pm sub, does $self->is_make_type('dmake') return true for you ?
In essence, you just need to modify that sub so that it returns \\\\ (4 backslashes) for you. Even if you just do it as:

sub init_DIRFILESEP {
     return '\\\\';
}
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That should do it.

Cheers,
Rob

Thanks for the help and insight, Rob. Both you and Eliya got me learning perl make Module internals, and I even learned a new idiom. I absolutely appreciate the effort you've shown in helping me.

However, it was the thought of having to create a kludge for a freshly installed Strawberry Perl that really got me looking closely at what Anonymous was writing about cpan conf options, and that lead to a solution my neat-freak mentality preferred.