but what you want to do is quite simple: ...

Hm. The only reference in Makefile.pl to libpq the following which comes after WriteMakeFile() has been called, and is used to check if it was detected (It is; I get no errors):

my $output = WriteMakefile(%opts);
if (!exists $output->{EXTRALIBS} or
    ($output->{EXTRALIBS} !~ /\-lpq/ and $output->{EXTRALIBS} !~ /libp
+q/)) {
    my $makefile = exists $output->{MAKEFILE}
        ? "\nRemoving ($output->{MAKEFILE})\n" : '';
    warn qq{
==========================================================
WARNING! No libpq libraries were detected!
You need to install the postgresql-libs package for your system,
or set the POSTGRES_LIB environment variable to the correct place.
$makefile
===========================================================
};
    ## Do not let make proceed
    unlink $output->{MAKEFILE} if $makefile;
    exit 1;
}
exit 0;
[download]

And the only references to the key LIBS in %opts as passed to WriteMakeFile() are these:

     LIBS           => ["-L$POSTGRES_LIB -lpq -lm"],
...
elsif ($os =~ /Win32/) {
    my $msdir = $POSTGRES_LIB;
    $msdir =~ s{"$}{/ms"};
    $opts{LIBS}[0] .= " -L$msdir -lsecur32";
[download]

So, I guess the "subtle point to my question" is how (and where/when) does -lpq get manipulated to cause it to add the reference to -llibpq?

On the basis that it EU::MM might simply generate references to all .libs it finds in the directory denoted by $ENV{ POSTGRES_LIB }, I tried placing a copy of libpq64.lb there, but it is ignored. As are the other .lib files that are located there: postgres.lib, libpgport.lib; libecpg.lib;.

So, something, somewhere, is telling EU::MM to generate references to libpq.lib, but I cannot see where or when that is occuring.

The second part of my question was how to automate the choice of whether to use libpq.lib or libpq64.lib. Your pseudo-code above neatly dodges both parts of that question.

• What goes inside your running_on_64bit()?

  Ie. How to reliably determine that the target is X64 (on Windows).

• Where in the makefile.pl would I position your pseudo-code?

  Given I can't see where libpq.lib is being referenced (other than in the post-facto test), it's hard to see where I would incorporate your code.