'foo' =~ /(foo)/;
length $3;
EXPECT
-Use of uninitialized value in length at - line 4.
+Use of uninitialized value $3 in length at - line 4.
########
# mg.c
use warnings 'uninitialized';
length $3;
EXPECT
-Use of uninitialized value in length at - line 3.
+Use of uninitialized value $3 in length at - line 3.
+########
+# mg.c
+use warnings 'uninitialized';
+$ENV{FOO} = undef; # should not warn
+EXPECT