diff --git a/src/test/perl/README b/src/test/perl/README
index 9e870830cf..fbb6e9384d 100644
--- a/src/test/perl/README
+++ b/src/test/perl/README
@@ -126,3 +126,7 @@ Due to limitations of cpanm, this recipe doesn't exactly duplicate the
 module list of older buildfarm animals.  The discrepancies should seldom
 matter, but if you want to be sure, bypass cpanm and instead manually
 install the desired versions of Test::Simple and IPC::Run.
+
+When passing a path from the Perl test code to PostgreSQL, like for example
+the path to an SSL certificate, it must be converted for use on the host
+system by using PostgreSQL::Test::Utils::perl2host.