Scalar::Util (distributed with 5.8) provides access to perl's internal function looks_like_number for determining whether a variable looks like a number.

update

despite older discussions recommending that way I do have problems on my system:

  DB<243> $nan=sin(9**9**9)
 => "nan"

  DB<244>  looks_like_number($nan)
 => 8704

  DB<245> $nan eq "nan"
 => 1

  DB<246>  Scalar::Util::looks_like_number($nan)
 => 36
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I know that NaN is plattform/C-compiler depended, so better test which way works.

update

according to perlop#Equality Operators

  DB<106> $nan=sin(9**9**9)
 => "nan"

  DB<107> print "NaN support here" if $nan != $nan
 => 1
NaN support here
  DB<108>
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