In op.c there is specific code to allow for keys( bareword ) to be treated as keys( %bareword ). If you turn on warnings you will get: Hash %bareword missing the % in argument 1 of keys() which is how I found the code: 

        case OA_HVREF:
            if (kid->op_type == OP_CONST &&
                (kid->op_private & OPpCONST_BARE))
            {

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and this makes me think that this is for compatability to something prior to Perl4.

Add to this the long-standing habit of print( hello ) being the same as print( "hello" ) (other than the optional errors and warnings).

Add to that a patch that noted that Module->Method() can sometimes be interpretted as Module()->Method() (or was it only Method Module that had that problem?) and that added support for the syntax Module::->Method() (or was it Method Module::?). I suspect that this is what made print( hello:: ) the same as print( "hello" ).

So these (may) explain why keys( main ) and keys( main:: ) are the same as keys( %main ).

Also consider that %main:: didn't use to work. You used to have to do %{"main::"}. And print( :: ) is the same as print( "::" ). So it makes some quirky sense that keys( :: ) is the same as keys( %:: ) which is now the same as keys( %{"main::"} ).

        - tye (those who don't know history are doomed to invent it)

In reply to (tye)Re: Why is (keys ::) legal syntax? by tye
in thread Why is (keys ::) legal syntax? by John M. Dlugosz

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