You can get the xor method to work on large numeric values by changing the swap function to stringify the values first:

sub xor { $a = "$a" ^ "$b"; $b = "$a" ^ "$b"; $a = "$a" ^ "$b"; }

$a=999999999999;
$b=8888888;
print "BEFORE: A=$a B=$b\n";
&xor();
print "AFTER:  A=$a B=$b\n\n";
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output

BEFORE: A=999999999999 B=8888888
AFTER:  A=8888888 B=999999999999