I was thinking about that! Convert to a string, and look to see if the first char is a "-" to determine sign. I don't think your routine works, though.

sub re {
  my $i=0; /-(?{$i=$i+1})/ for @_; $i=~/0|2/;
}
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given the same test cases as before,

x	y	xorNeg()	spaceship()	mult()	anchor()	re()
1	1		1	1	1	1
-1	1				0	1
1	-1				0	1
-1	-1		1	1	1	1

And actually, its only twice as slow...

unit square	spaceship	multiply	logic	re
spaceship	--------	worse 21.74%	worse 12.21%	better 86.26%
multiply	better 21.74%	--------	better 10.86%	better 89.25%
logic	better 12.21%	worse 10.86%	--------	better 87.94%
reg exp	worse 86.26%	worse 89.25%	worse 87.94%	--------