From the perlfunc::open section of the site(aslo the perl manpages):

If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename.

That means that if an open is called like open 0 then 0 is the filehandle, and the "scalar variable" to open. Take, for instance, this example from perlfunc::open:

$ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...
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If your asking why, as in why is it that way, I'd presume its for brevity and ease, as in:

$LOG = '/path/to/log/file/';
open LOG;
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