hopefully someone will be able to come up with a updated version for all Perl users that works just as well

The thing is that perl users might not all agree on what IsNumber() should return for all inputs.
It can often depend upon what the intended usage is.

A good starting point is probably to use Scalar::Util::looks_like_number() and then tweak the results to allow for any cases where that function's return does not suit your needs.
For example:

>perl -MScalar::Util="looks_like_number" -we "print 'WTF' if looks_lik
+e_number('0xa.8');"

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That's probably what you want since perl is only going to numify that string to 0. But if you're going to pass the string '0xa.8' to a function that handles such an input as intended, then you'd want your IsNumber() subroutine to return TRUE.

>perl -MMath::MPFR -wle "print Math::MPFR->new('0xa.8');"
1.05e1

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Other points of contention might be whether your IsNumber() function should return true for numeric objects like Math::BigInt, Math::BigFloat and Math::BigRat objects ... or maybe you don't even have to consider such inputs.

Cheers,
Rob