AHA! bgreenlee, you have helped me resolve my mystery. Here's what happened. I originally tested the results like this:

my $a = 3.0;
my $b = '3.0';

my $stringCompareResult = $a eq $b?'true':'false';
my $numberCompareResult = $a == $b?'true':'false';

sub is_numeric {
  ($_[0] & ~ $_[0]) eq "0";
}

print ("\$stringCompareResult: $stringCompareResult\n");
print ("\$numberCompareResult: $numberCompareResult\n");

my $isNumericResultA = is_numeric($a)?'true':'false';
my $isNumericResultB = is_numeric($b)?'true':'false';
print ("\$isNumericResultA: $isNumericResultA\n");
print ("\$isNumericResultB: $isNumericResultB\n");
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When I ran this code, I got the following output:

$stringCompareResult: false
$numberCompareResult: true
$isNumericResultA: true
$isNumericResultB: true
[download]

I see, now, thanks to your reply, that when I used $a and $b in the earlier comparison, the interpreter CHANGED their "type". I had forgetting that Perl does this.

When I comment out the comparisons, and run just is_numeric, I get the same results as you. Thanks, bgreenlee!