my @tests = ("1 + 2", "2*3", "(2 + 5) / 3", 
             "print 'a test', "E = m * c**2", "3**2 + 2");

for my $expr (@tests) {

    # Does the string contain any characters that we
    # don't want?

    if ($expr =~ m![^0-9 + \- / * \( \) \s]!x) {
        print "$expr: Invalid\n";
    } else {
        print "$expr = ", eval $expr, "\n";
    }
}