Here are some examples on how to test for error messages using CPAN modules.

Check that code does or does not die (with a specific error message):

use Test::Exception;

dies_ok { testsub("die") } "Dies on inappropriate input";
throws_ok { testsub("die screaming") } qr/arrrgh/, "Dies with appropri
+ate error message";
lives_ok { testsub("survive") } "Survives with appropriate input";
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Check that a warning is or is not given:

use Test::Warn;

warning_is { testsub("complain") } qr/nag/, "Gives a specific warning"
+;
warning_is { testsub("quiet") undef, "No warnings given";
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Check that none of the tests you run emit a warning:

use Test::NoWarnings;
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For this last one you'll have to add 1 to the number of tests you run (in your Test::More plan), which will then pass whether any of the tests in this file emitted a warning.

See the perldoc for each of these modules for further examples and documentation.