$ perlbrew exec perl -e 'for(scalar($#foo)) { $_=3 } warn "$] ".@foo."\n"' >/dev/null
5.034000 4
5.032001 4
5.030003 4
5.028003 4
5.026003 4
5.024004 4
5.022004 4
5.020003 0
5.018004 0
$ perlbrew exec perl -e 'for(~~$#foo) { $_=3 } warn "$] ".@foo."\n"' >/dev/null
5.034000 0
5.032001 0
5.030003 0
5.028003 0
5.026003 0
5.024004 0
5.022004 0
5.020003 0
5.018004 0

##</code><code>##

$ perl -le 'print scalar($#foo)'
-1
$ perl -le 'print ~~$#foo'
18446744073709551615