diff --git a/var/spack/repos/builtin/packages/mfem/package.py b/var/spack/repos/builtin/packages/mfem/package.py
index 667d07efe4e..2b74a154870 100644
--- a/var/spack/repos/builtin/packages/mfem/package.py
+++ b/var/spack/repos/builtin/packages/mfem/package.py
@@ -601,7 +601,7 @@ def find_optional_library(name, prefix):
             else:
                 cxxstd_flag = getattr(self.compiler, "cxx" + cxxstd + "_flag")
 
-        cxxflags = spec.compiler_flags["cxxflags"]
+        cxxflags = spec.compiler_flags["cxxflags"].copy()
 
         if cxxflags:
             # Add opt/debug flags if they are not present in global cxx flags
@@ -1116,7 +1116,7 @@ def patch(self):
             "miniapps/gslib/findpts.cpp",
             "miniapps/gslib/pfindpts.cpp",
         ]
-        bom = "\xef\xbb\xbf" if sys.version_info < (3,) else u"\ufeff"
+        bom = "\xef\xbb\xbf" if sys.version_info < (3,) else "\ufeff"
         for f in files_with_bom:
             filter_file(bom, "", f)