The bitwise noop 0 | $foo normally forces unsignedness, but use integer; interferes with that, too.The no integer; pragma has lexical scope, so you can wrap it up in a bareblock with the unsigned operations.

perl -Minteger -e'my $foo = 2**63-1 ; $foo = 2*$foo;print $foo,$/; { n
+o integer; print 0|$foo,$/;}'
-2
18446744073709551614
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After Compline,
Zaxo