my $inf = 9**9**9;
my $neginf = -9**9**9;
my $nan = -sin(9**9**9);

sub isinf { $_[0]==9**9**9 || $_[0]==-9**9**9 }
sub isnan { ! defined( $_[0] <=> 9**9**9 ) }
# useful for detecting negative zero
sub signbit { substr( sprintf( '%g', $_[0] ), 0, 1 ) eq '-' }

for my $num ( $inf, $neginf, $nan ) {
   printf("%s:tisinf: %d,tisnan: %d,tsignbit: %dn", $num, isinf($num), isnan($num), signbit($num));
}

inf:    isinf: 1,   isnan: 0,   signbit: 0
-inf:   isinf: 1,   isnan: 0,   signbit: 1
nan:    isinf: 0,   isnan: 1,   signbit: 0

print "Is NaNn" if $a eq 'nan';
print "Is Infn" if $a eq 'inf';