my $sub_to_test = "test_sub";   

# try to fire an EVAL_ERROR
eval "$sub_to_test()";                          

# if no EVAL_ERROR
if (!$@) {                                      
        # do the thing
        eval "$sub_to_test ('testing, 1,2,3')"; 
}

sub test_sub {
        return unless @_;
        print "TEST_SUB: $_[0]\n";
}