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  sub atoi {
    my $t;
...
    }
  }
  $number = atoi("123");

  $string = sprintf("%f", 123.45);

   if ($string == "$string") { 
   # It is a number
   }

  if ($var =~ /(?=.)M{0,3}(C[MD]|D?C{0,3})(X[CL]|L?X{0,3})(I[XV]|V?I{0
+,3})/) {
    print "$var contains a number.\b";
  }