Adding use encoding "iso-8859-1"; (to explicitly tell Perl that your source is in iso-8859-1) at the top of your module did fix it for me: 

$ make test
PERL_DL_NONLAZY=1 /usr/local/perl/5.10.0/bin/perl "-MExtUtils::Command
+::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/Lingua-Stem-Es....ok                                                
+       
All tests successful.
Files=1, Tests=28379, 11 wallclock secs ( 8.21 cusr +  0.30 csys =  8.
+51 CPU)

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In reply to Re: RegExp breaks in Perl 5.10 by almut
in thread RegExp breaks in Perl 5.10 by jfraire

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