grep applies a test to all the elements of a list and creates a subset of the elements that pass the test. Using it in scalar context return the number of elements which passed the test.

However, if you know the username first, I would suggest making the database do the work and re-writing the query to something like this (note that my SQL skills are quite rusty):

$sth = $dbh -> prepare(q{select username, password from reg where user
+name = $testUsername});
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Why return a list to Perl and check the entire list for the existence of username when the database can do exactly that? If you're testing a large number of user names, it may make more sense to query the database for a list, and then check the list for the for existence of all the user names(subject to the caution that the database may have been updated during the processing of the list).