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$[++; # No go.
$[+=1; # Not happening
$[ = $[; # Works.
...
print \$[; # Ok, works.
$[ = \$[; # which also works
print $[; # Whoo fun!

$[=-1;
@_=qw/foo bar baz/;
print $_[-1]; # foo
print $_[-2]; # bar
print $_[-3]; # foo

#WRONG:
for(my $i=0; $i<$#foo; $i++){}
#RIGHT:
for(my $i=$[; $i<$#foo; $i++){}

#WRONG:
foreach(0..$#foo){}
#RIGHT:
foreach($[..$#foo){}