diff --git a/config/tcl.m4 b/config/tcl.m4
index a9fe7ba5cc1..4010eb42eab 100644
--- a/config/tcl.m4
+++ b/config/tcl.m4
@@ -13,6 +13,10 @@ fi
 
 # PGAC_PATH_TCLCONFIGSH([SEARCH-PATH])
 # ------------------------------------
+# If the user doesn't specify $TCL_CONFIG_SH directly, search for it in
+# the list of directories passed as parameter (from --with-tclconfig).
+# If no list is given, try the Tcl shell's $auto_path.
+
 AC_DEFUN([PGAC_PATH_TCLCONFIGSH],
 [AC_REQUIRE([PGAC_PATH_TCLSH])[]dnl
 AC_BEFORE([$0], [PGAC_PATH_TKCONFIGSH])[]dnl
@@ -24,7 +28,14 @@ if test -z "$TCL_CONFIG_SH"; then
     set X $pgac_test_dirs; shift
     if test $[#] -eq 0; then
         test -z "$TCLSH" && AC_MSG_ERROR([unable to locate tclConfig.sh because no Tcl shell was found])
-        set X `echo 'puts $auto_path' | $TCLSH`; shift
+        pgac_test_dirs=`echo 'puts $auto_path' | $TCLSH`
+        # On newer macOS, $auto_path frequently doesn't include the place
+        # where tclConfig.sh actually lives.  Append that to the end, so as not
+        # to break cases where a non-default Tcl installation is being used.
+        if test -d "$PG_SYSROOT/System/Library/Frameworks/Tcl.framework" ; then
+            pgac_test_dirs="$pgac_test_dirs $PG_SYSROOT/System/Library/Frameworks/Tcl.framework"
+        fi
+        set X $pgac_test_dirs; shift
     fi
 
     for pgac_dir do
diff --git a/configure b/configure
index 4030e8d9ef0..7d111323960 100755
--- a/configure
+++ b/configure
@@ -683,6 +683,7 @@ python_majorversion
 PYTHON
 perl_embed_ldflags
 perl_embed_ccflags
+perl_includedir
 perl_useshrplib
 perl_privlibexp
 perl_archlibexp
@@ -7791,6 +7792,14 @@ perl_useshrplib=`$PERL -MConfig -e 'print $Config{useshrplib}'`
 test "$PORTNAME" = "win32" && perl_useshrplib=`echo $perl_useshrplib | sed 's,\\\\,/,g'`
 { $as_echo "$as_me:$LINENO: result: $perl_useshrplib" >&5
 $as_echo "$perl_useshrplib" >&6; }
+  # On most platforms, archlibexp is also where the Perl include files live ...
+  perl_includedir="$perl_archlibexp"
+  # ... but on some macOS versions, we must look under $PG_SYSROOT instead
+  if test x"$PG_SYSROOT" != x"" ; then
+    if test -d "$PG_SYSROOT$perl_archlibexp" ; then
+      perl_includedir="$PG_SYSROOT$perl_archlibexp"
+    fi
+  fi
 
 { $as_echo "$as_me:$LINENO: checking for CFLAGS recommended by Perl" >&5
 $as_echo_n "checking for CFLAGS recommended by Perl... " >&6; }
@@ -30212,7 +30221,14 @@ if test -z "$TCL_CONFIG_SH"; then
         test -z "$TCLSH" && { { $as_echo "$as_me:$LINENO: error: unable to locate tclConfig.sh because no Tcl shell was found" >&5
 $as_echo "$as_me: error: unable to locate tclConfig.sh because no Tcl shell was found" >&2;}
    { (exit 1); exit 1; }; }
-        set X `echo 'puts $auto_path' | $TCLSH`; shift
+        pgac_test_dirs=`echo 'puts $auto_path' | $TCLSH`
+        # On newer macOS, $auto_path frequently doesn't include the place
+        # where tclConfig.sh actually lives.  Append that to the end, so as not
+        # to break cases where a non-default Tcl installation is being used.
+        if test -d "$PG_SYSROOT/System/Library/Frameworks/Tcl.framework" ; then
+            pgac_test_dirs="$pgac_test_dirs $PG_SYSROOT/System/Library/Frameworks/Tcl.framework"
+        fi
+        set X $pgac_test_dirs; shift
     fi
 
     for pgac_dir do
@@ -30393,7 +30409,7 @@ fi
 # check for <perl.h>
 if test "$with_perl" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$CPPFLAGS -I$perl_archlibexp/CORE"
+  CPPFLAGS="$CPPFLAGS -I$perl_includedir/CORE"
   { $as_echo "$as_me:$LINENO: checking for perl.h" >&5
 $as_echo_n "checking for perl.h... " >&6; }
 if test "${ac_cv_header_perl_h+set}" = set; then
diff --git a/configure.in b/configure.in
index bce1b2afe35..1511108d470 100644
--- a/configure.in
+++ b/configure.in
@@ -880,6 +880,15 @@ if test "$with_perl" = yes; then
     AC_MSG_ERROR([Perl not found])
   fi
   PGAC_CHECK_PERL_CONFIGS([archlibexp,privlibexp,useshrplib])
+  # On most platforms, archlibexp is also where the Perl include files live ...
+  perl_includedir="$perl_archlibexp"
+  # ... but on some macOS versions, we must look under $PG_SYSROOT instead
+  if test x"$PG_SYSROOT" != x"" ; then
+    if test -d "$PG_SYSROOT$perl_archlibexp" ; then
+      perl_includedir="$PG_SYSROOT$perl_archlibexp"
+    fi
+  fi
+  AC_SUBST(perl_includedir)dnl
   PGAC_CHECK_PERL_EMBED_CCFLAGS
   PGAC_CHECK_PERL_EMBED_LDFLAGS
 fi
@@ -1892,7 +1901,7 @@ fi
 # check for <perl.h>
 if test "$with_perl" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$CPPFLAGS -I$perl_archlibexp/CORE"
+  CPPFLAGS="$CPPFLAGS -I$perl_includedir/CORE"
   AC_CHECK_HEADER(perl.h, [], [AC_MSG_ERROR([header file <perl.h> is required for Perl])],
                   [#include <EXTERN.h>])
   # While we're at it, check that we can link to libperl.
diff --git a/src/Makefile.global.in b/src/Makefile.global.in
index db40212f058..a439b200c5c 100644
--- a/src/Makefile.global.in
+++ b/src/Makefile.global.in
@@ -285,6 +285,7 @@ else
 endif
 perl_archlibexp		= @perl_archlibexp@
 perl_privlibexp		= @perl_privlibexp@
+perl_includedir		= @perl_includedir@
 perl_useshrplib		= @perl_useshrplib@
 perl_embed_ccflags	= @perl_embed_ccflags@
 perl_embed_ldflags	= @perl_embed_ldflags@
diff --git a/src/pl/plperl/GNUmakefile b/src/pl/plperl/GNUmakefile
index 32d49cdaa15..7559d4565d0 100644
--- a/src/pl/plperl/GNUmakefile
+++ b/src/pl/plperl/GNUmakefile
@@ -26,7 +26,7 @@ endif
 # probably because it sometimes contains some header files with names
 # that clash with some of ours, or with some that we include, notably on
 # Windows.
-override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS) $(perl_embed_ccflags) -I$(perl_archlibexp)/CORE
+override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS) $(perl_embed_ccflags) -I$(perl_includedir)/CORE
 
 rpathdir = $(perl_archlibexp)/CORE
 
diff --git a/src/template/darwin b/src/template/darwin
index 542f706b0fa..cff258be26b 100644
--- a/src/template/darwin
+++ b/src/template/darwin
@@ -1,9 +1,24 @@
 # src/template/darwin
 
-# Select appropriate semaphore support.  Darwin 6.0 (Mac OS X 10.2) and up
-# support System V semaphores; before that we have to use POSIX semaphores,
-# which are less good for our purposes because they eat a file descriptor
-# per backend per max_connection slot.
+# Note: Darwin is the original code name for macOS, also known as OS X.
+# We still use "darwin" as the port name, partly because config.guess does.
+
+# Select where system include files should be sought.
+if test x"$PG_SYSROOT" = x"" ; then
+  PG_SYSROOT=`xcodebuild -version -sdk macosx Path 2>/dev/null`
+fi
+if test x"$PG_SYSROOT" != x"" ; then
+  if test -d "$PG_SYSROOT" ; then
+    CPPFLAGS="$CPPFLAGS -isysroot $PG_SYSROOT"
+  else
+    PG_SYSROOT=""
+  fi
+fi
+
+# Select appropriate semaphore support.  Darwin 6.0 (macOS 10.2) and up
+# support System V semaphores; before that we have to use named POSIX
+# semaphores, which are less good for our purposes because they eat a
+# file descriptor per backend per max_connection slot.
 case $host_os in
   darwin[015].*)
     USE_NAMED_POSIX_SEMAPHORES=1