A Boolean context doesn't have to take only a comparison (==, !=, <, >=, etc.). Any value (including the value returned by LastError()) can be tested for truth. perldoc perldata says:

A scalar value is interpreted as TRUE in the Boolean sense if it is not the null string or the number 0 (or its string equivalent, "0"). The Boolean context is just a special kind of scalar context where no conversion to a string or a number is ever performed.

See What is true and false in Perl? for a longer discussion.