> whereas this version would need to change to ... <= 0

IMHO for a multiplying approach that's the best check, even for the <=> version.

Simply changing to ... < 0 would check for the open interval excluding the endpoints. And ... == 0 for endpoints only.

But half-open intervals like [$a,$b[ can't be covered with this approach.

So I'd rather stick with

• $a <= $x and $x < $b

or the newer chained version

• $a <= $x < $b

and make sure the endpoints are previously swapped if necessary

• ($a,$b) = ($b,$a) if $b < $a;

update

and swapping can be made non-destructive by localizing it to a scope.

C:\tmp\e>perl
($a,$b) = (7,3);
my $x=5;
my $inside = do {
    local ($a,$b) = ($b,$a) if $b < $a;
    $a <= $x < $b ;
};
print "$x in [$a, $b]? $inside";
__END__
5 in [7, 3]? 1
C:\tmp\e>
[download]

(replace local with my for private vars)