if ($b == 1 || $b == 2 || $b == 3 || $b == 4 || $b == 5) {
   print "\$b is a number and is either 1, 2, 3, 4, or 5.";
} else {
   print "\$b has to be a number and to be either 1, 2, 3, 4, or 5.";
}

##</code><code>##

if ($b =~ /^\d$/ and $b > 0 and $b < 6) {
    ...
}