diff --git a/Mk/bsd.perl.mk b/Mk/bsd.perl.mk
index 6319b24286d1..c8a0e1651e3f 100644
--- a/Mk/bsd.perl.mk
+++ b/Mk/bsd.perl.mk
@@ -73,6 +73,8 @@
 #				  port.  The version requirement can be specified
 #				  here, as well.
 #
+# WANT_PERL5	- Set this is your port conditionally depends on Perl.
+#				  This MUST appear before the inclusion of bsd.port.pre.mk.
 
 .if !defined(_POSTMKINCLUDED) && !defined(Perl_Pre_Include)
 
@@ -182,6 +184,10 @@ PLIST_SUB+=	PERL_VERSION=${PERL_VERSION} \
 
 .endif  # defined(_PERL_REFACTORING_COMPLETE)
 
+.if !defined(PERL) || !defined(PERL5) || !defined(PERL_PORT) || !defined(PERL_VERSION)
+IGNORE=		missing define for WANT_PERL, USE_PERL5, or similar before bsd.port.pre.mk inclusion
+.endif
+
 .if defined(PERL_MODBUILD)
 PERL_CONFIGURE=		yes
 CONFIGURE_SCRIPT?=	Build.PL