fixed a bizarre error hiding condition

turned out to be necessary to localize $@ in Parser::split_proto()
also finally came up with a test case which demonstrates the error

lib/Method/Signatures/Parser.pm

@@ -12,6 +12,8 @@ sub split_proto {
     my $proto = shift;
     return unless $proto =~ /\S/;
 
+    local $@ = undef;
+
     require PPI;
     my $ppi = PPI::Document->new(\$proto);
     $ppi->prune('PPI::Token::Comment');

t/error_interruption.t

@@ -0,0 +1,14 @@
+
+use strict;
+use warnings;
+
+use lib 't/lib';
+
+use Test::More;
+use Test::Exception;
+
+
+throws_ok { require BarfyDie } qr/requires explicit package name/, "MS doesn't interrupt real compilation error";
+
+
+done_testing();

t/lib/BarfyDie.pm

@@ -0,0 +1,25 @@
+# For use with t/error_interruption.t
+
+package BarfyDie;
+
+use strict;
+use warnings;
+
+use Method::Signatures;
+
+
+# This _should_ produce a simple error like the following:
+# Global symbol "$foo" requires explicit package name at t/lib/BarfyDie.pm line 13.
+$foo = 'hi!';
+
+# And, without the signature below, it would.
+# For that matter, if you compile this by itself, it still does.
+# However, when you require this file from inside an eval, Method::Signature's parser() method will
+# eat the error unless we localize $@ there.  So this verifies that we're doing that.
+
+method foo (Str $bar)
+{
+}
+
+
+1;