diff --git a/src/perl/perl-core.c b/src/perl/perl-core.c
index 59ca58fe..67fff292 100644
--- a/src/perl/perl-core.c
+++ b/src/perl/perl-core.c
@@ -18,7 +18,9 @@
     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <wchar.h>
 
 #define NEED_PERL_H