Unary "-" performs arithmetic negation if the operand is
       numeric.  If the operand is an identifier, a string
       consisting of a minus sign concatenated with the
       identifier is returned.  Otherwise, if the string starts
       with a plus or minus, a string starting with the opposite
       sign is returned.  One effect of these rules is that
       -bareword is equivalent to "-bareword".
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Which is why -beer isn't a bareword. But the => is equivalent to a , (comma) thus ending the bareword, er string. So then
    -beer, none
will result in none being a bareword. At least, that's how I understand it.

You are correct of course.
non strict code:

&a(-beer=>none);
sub a
{
    print join("=>",@_);
}
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This will product the correct results of "-beer=>none" as long as you are not using 'strict' or -w, but of course nobody does that ... right? So to be proper:

use strict;
&a(-beer=>'none');
sub a
{
    print join("=>",@_);
}
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and this will product the same results, with no warnings.