Sorry I can't help more

That's ok - you've alrready gone to more trouble than is warranted.

There seems to be a fundemental conflict between an XS extension to call functions in a file called gmp.dll, itself being called gmp.dll

Yes (on Win32 anyway), the dll that perl builds needs to have a different name to the the third party dll. I struck a similar problem trying to build PGPLOT. One workaround is to have perl build a dll with a different extension by specifying in WriteMakefile() something like: 
    DLEXT => 'xs.dll',

[download]
The bootstrap() call in GMP.pm then also needs to be changed: 
if($^O =~ /mswin32/i) {
  local $DynaLoader::dl_dlext = 'xs.dll';
  bootstrap Math::GMP $VERSION;
  }

else {bootstrap Math::GMP $VERSION}

[download]
That should result in perl building a dll called gmp.xs.dll - which won't get confused with the gmp.dll that you already have.

.. why do so few of these modules that have dependancies on third party libraries never bother to give pointers to where to get them, or even mention the dependancy

Good question - and I can't really answer for Math::GMP. In my view, that info should be provided in the README - and I have GMP-based modules that do that. I guess it wouldn't hurt to duplicate that info in the POD - which I will do.

Cheers,
Rob

In reply to Re^6: Writing threadsafe perl extensions by syphilis
in thread Writing threadsafe perl extensions by syphilis

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