You're over thinking it.

my $x = "0.01 NaN 2.30 4.44";

Match the form rather than the content of the data:

my $r1 = qr/(\S+)\s+(\S+)\s+(\S+)\s+(\S+)/;

Or ditch the pattern and split, as others suggest:

my ($d, $e, $f, $g) = split /\s+/, $x;