my @lib_flags = qw(this that other);

if (running_on_64bit()) {
    push @lib_flags, 'libpq64.lib';
}
else {
    push @lib_flags, 'libpq.lib';
}

# ... time passes

WriteMakefile(
    NAME => 'DBD::Pg',
    LIBS => ["@LIB_FLAGS"],
    ...
);