Any idea how to have it working with INST_ARCH set?

I'm seeing the same as you. It's buggy.
I suspect that the escaping of $(ARCHNAME) is incorrect, and that the backslash needs to be removed: 
INST_ARCH    := $(ARCHNAME)

[download]
At least it will then build current blead (perl-5.31.7). But it seems to produce the same structure as a perl built without INST_ARCH.
So I then additionally tried supplying it with the archname that needs to be used for the particular build I was doing - ie ARCHNAME=MSWin32-x64-multi-thread But that just resulted in an installation whose top level directory structure was quite bizarre: 
bin
binMSWin32-x64-multi-thread
html
lib
libMSWin32-x64-multi-thread
site

[download]
and an empty @INC (as you noted).
So ... there's a bit to unravel, and the suggestion of building with dmake and makefile.mk looks the best short term option.

Cheers,
Rob

In reply to Re: Perl compilation on Windows with INST_ARCH set fails by syphilis
in thread Perl compilation on Windows with INST_ARCH set fails by ivpri

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.