if ($my_dat =~ /[^0-9]/) {
   # $my_dat contains a non-digit.
   ...
}

##</code><code>##

if ($my_dat =~ /^[0-9]+$/) {
   # $my_dat only contains digits.
   ...
}

##</code><code>##

if ($my_dat =~ /^[0-9]{2}$/) {
   # $my_dat only contains digits.
   # $my_dat is exactly two characters long.
   ...
}