Just to elaborate a bit more on this, consider the code:

my @PP = ("foo", "bar", "baz");
my @OO = ("AAA", "bar", "CCC");

$PP[1][0] = "foobar";
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What happens is that this does not modify the @PP array, instead it uses the value of $PP[1] as an array reference which points to the global(/package) variable @bar. To make this a bit clearer:

#!/usr/bin/perl -l

my @PP = ("foo", "bar", "baz");
my @OO = ("AAA", "bar", "CCC");

$PP[1][0] = "foobar";

print "\@PP: " . join(", ", @PP);
print "\@OO: " . join(", ", @OO);
print "\@bar: " . join(", ", @::bar);
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Output:

@PP: foo, bar, baz
@OO: AAA, bar, CCC
@bar: foobar

The '@PP' and '@OO' array are unmodified; the '@bar' array however was created.

With use strict; you get the error: Can't use string ("bar") as an ARRAY ref while "strict refs" in use