# Test::Simple or Test::More
ok($var eq 'a' || $var eq 'b', "test name");
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Update: map will help you avoid evaluating the expression twice. (Be careful about context.)
( Oops, added missing parens. Thanks shenme )

# Test::Simple or Test::More
ok((map { $_ eq 'a' || $_ eq 'b' } $var), "test name");
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Update: Or you could use a regexp.

# Test::More
like($var, qr/^(?:a|b)\z/, "test name");
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