my $price = "100.09";

if ($price =~ / ^      # We begin at the start of the string
                [0-9]+ # At least one digit, possibly followed by more
                (      # Then (possibly, see below) follows
                  \.   # a dot
                  [0-9][0-9] # and two more digits
                )?     # But the decimal part is optional
                $      # And that's all there is to be
              /x) {

} elsif {
  die "'$price' dosen't look like a valid price.\n"
};