diff --git a/build/build.pl b/build/build.pl
index 6b0d78f..07dea00 100755
--- a/build/build.pl
+++ b/build/build.pl
@@ -92,8 +92,16 @@ my $needs_reconfig = !!$opts{reconfig};
 	# Emscripten's fork() (and system()) stubs return EAGAIN, meaning "Resource temporarily unavailable".
 	# So perl will wait 5 seconds and try again, which is not helpful to us, since Emscripten doesn't support those functions at all.
 	# This patch fixes that on the Emscripten side, so the stubs return ENOTSUP.
+	# first, we need to take a guess which version of the patch to apply.
+	my $libraryjs = file($ENV{EMSCRIPTEN}, 'src', 'library.js')->slurp;
+	my $patchf;
+	if ( $libraryjs=~/\b\QERRNO_CODES.EAGAIN\E\b/ )
+		{ $patchf = 'emscripten_1.38.10_eagain.patch' }
+	elsif ( $libraryjs=~/\b\QcDefine('EAGAIN')\E/ )
+		{ $patchf = 'emscripten_1.38.28_eagain.patch' }
+	else { die "Could not figure out which library.js patch to use" }
 	#TODO Later: we should probably verify the Emscripten version too, and in the future we may need different patches for different versions
-	if ( try_patch_file( file($FindBin::Bin,'emscripten_1.38.10_eagain.patch') ) ) {
+	if ( try_patch_file( file($FindBin::Bin,$patchf) ) ) {
 		say STDERR "# Emscripten was newly patched, forcing a rebuild";
 		# not sure if the following is needed, but playing it safe:
 		run 'emcc', '--clear-cache';  # force Emscripten to rebuild libs (takes a bit of time)
diff --git a/build/emperl_config.sh b/build/emperl_config.sh
index 8aa4cd0..9bc9562 100755
--- a/build/emperl_config.sh
+++ b/build/emperl_config.sh
@@ -29,13 +29,18 @@ export EMPERL_OUTPUTDIR="$BASEDIR/work/outputperl"
 # Note to self: In build.pl, we take advantage of the fact that on Perls >=v5.10.0, "$^V" is the same as the tag name.
 export EMPERL_PERLVER="v5.28.0"
 export EMPERL_PREFIX="/opt/perl"
+# Note: strace shows this is how file_packager.py is called: ["/usr/bin/python", "/home/haukex/emsdk/emscripten/1.38.28/tools/file_packager.py", "emperl.data", "--from-emcc", "--export-name=Module", "--preload", "/home/haukex/code/webperl/work/outputperl/opt/perl@/opt/perl", "--no-heap-copy"]
 export EMPERL_PRELOAD_FILE="$EMPERL_OUTPUTDIR$EMPERL_PREFIX@$EMPERL_PREFIX"
 export EMPERL_OPTIMIZ="-O2"
-export EMPERL_LINK_FLAGS="--pre-js common_preamble.js -s EXPORTED_FUNCTIONS=['_main','_emperl_end_perl','_Perl_call_sv','_Perl_call_pv','_Perl_call_method','_Perl_call_argv','_Perl_eval_pv','_Perl_eval_sv','_webperl_eval_perl'] -s EXTRA_EXPORTED_RUNTIME_METHODS=['ccall','cwrap']"
+# Note: We explicitly disable ERROR_ON_UNDEFINED_SYMBOLS because it was enabled by default in Emscripten 1.38.13.
+#TODO Later: Why does --no-heap-copy not get rid of the "in memory growth we are forced to copy it again" assertion warning? (https://github.com/emscripten-core/emscripten/commit/ec764ace634f13bab5ae932912da53fe93ee1b69)
+export EMPERL_LINK_FLAGS="--pre-js common_preamble.js --no-heap-copy -s ERROR_ON_UNDEFINED_SYMBOLS=0 -s EXPORTED_FUNCTIONS=['_main','_emperl_end_perl','_Perl_call_sv','_Perl_call_pv','_Perl_call_method','_Perl_call_argv','_Perl_eval_pv','_Perl_eval_sv','_webperl_eval_perl'] -s EXTRA_EXPORTED_RUNTIME_METHODS=['ccall','cwrap']"
+
+export EMPERL_DEBUG_FLAGS=""
+#export EMPERL_DEBUG_FLAGS="-s ASSERTIONS=2 -s STACK_OVERFLOW_CHECK=2"
 # Note: not including "-s SAFE_HEAP=1" in the debug flags because we're building to WebAssembly, which doesn't require alignment
 #TODO Later: Can some of the SAFE_HEAP functionality (null pointer access I think?) be replaced by the WASM error traps?
 # http://kripken.github.io/emscripten-site/docs/compiling/WebAssembly.html#binaryen-codegen-options
-export EMPERL_DEBUG_FLAGS="-s ASSERTIONS=2 -s STACK_OVERFLOW_CHECK=2"
 
 # Location and branch of the perl git repository that contains the emperl branch
 export EMPERL_PERL_REPO="https://github.com/haukex/emperl5.git"
diff --git a/build/emscripten_1.38.28_eagain.patch b/build/emscripten_1.38.28_eagain.patch
new file mode 100644
index 0000000..766e29b
--- /dev/null
+++ b/build/emscripten_1.38.28_eagain.patch
@@ -0,0 +1,20 @@
+--- library.js.orig	2019-03-02 16:08:24.404047130 +0100
++++ library.js	2019-03-02 16:19:30.588047130 +0100
+@@ -291,7 +291,7 @@
+     // pid_t fork(void);
+     // http://pubs.opengroup.org/onlinepubs/000095399/functions/fork.html
+     // We don't support multiple processes.
+-    ___setErrNo({{{ cDefine('EAGAIN') }}});
++    ___setErrNo({{{ cDefine('ENOTSUP') }}});
+     return -1;
+   },
+   vfork: 'fork',
+@@ -817,7 +817,7 @@
+     // int system(const char *command);
+     // http://pubs.opengroup.org/onlinepubs/000095399/functions/system.html
+     // Can't call external programs.
+-    ___setErrNo({{{ cDefine('EAGAIN') }}});
++    ___setErrNo({{{ cDefine('ENOTSUP') }}});
+     return -1;
+   },
+