(This post is substantially the same as what I said in the Chatterbox. I'm posting it here for future reference.)

[...] works, as does reading with <main::DATA>. Freaky.

Every module can have a __DATA__ section, so the file handle DATA is tied to a package.

But that's not special. While there are a few special file handles which are trully global (STDIN, STDOUT, STDERR and ARGV), all others are stored in lexical variables or package variables. For example,

package AA;
open(FILE, '<', $ARGV[0]) or die("open: $!\n");
package BB;
print(defined(read(FILE, $buf='', 0))?1:0, "\n");  # 0
package AA;
print(defined(read(FILE, $buf='', 0))?1:0, "\n");  # 1
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