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

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

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