9 characters long? Does 1.3456789 count? Or do you really mean 5 digits after the decimal place only? I'm assuming the latter. And that the - sign doesn't count.

  /^-?(\d{1,3}(?:\.\d{1,5})?)$/
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That is, an optional leading - sign, followed by 1 to 3 digits (to allow for 180 degrees East/West), followed, optionally, by a dot, and then 1 to 5 more digits.

Better yet, try:

  use Regexp::Common;
  /^$RE{num}{decimal}{-places=>'0,5'}$/ and $_ <= 180;
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No sense reinventing the wheel. :-)