Invoking MySQL without a username or a password defaults to the current login and no password:

perl -MDBI -we'my $dbh=DBI->connect("dbi:mysql:","","")'
DBI connect('','',...) failed: Access denied for user 'ben'@'localhost
+' (using password: NO) at -e line 1
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...and there doesn't seem to be any way to force it to accept a blank username, either:

mysql --user="" --password=""
ERROR 1045 (28000): Access denied for user 'ben'@'localhost' (using pa
+ssword: NO)
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-- 
Education is not the filling of a pail, but the lighting of a fire.
 -- W. B. Yeats