The special variables are "super global"*, accessible from any namespace without qualification. This is done by making the name of the variable "super global". That means that if $_ is super global, so is array @_, hash %_, sub &_, etc.

"ARGV" is one of those names. No matter what namespace is current, sub ARGV { } will define &main::ARGV, and ARGV() will call &main::ARGV.

You can get around that by specifying which package's ARGV you want.

{
   package Foo;
   sub ARGV      { print "1\n" }
   sub Foo::ARGV { print "2\n" }
}

{
   package Bar;
   ARGV();        # 1
   Foo::ARGV();   # 2
   Foo->ARGV();   # 2
}
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Replace "ARGV" with "_" and you'll get the same results.

* — Not a technical term :)