Things that can usually be used as a file handle:

• A reference to an IO object (e.g. *STDOUT{IO}).
• A glob containing a reference to an IO object (e.g. *STDOUT).
• A reference to a glob containing a reference to an IO object (e.g. \*STDOUT).
• A string naming a glob containing a reference to an IO object (e.g. "STDOUT").

I would expect functions (named operators) to accept all of these. Subs may be more picky. In particular, subs don't normally support the last one.

Language constructs (syntax) that can be usually be used to provide a file handle to a function (named operator):

• A bareword naming a glob (e.g. STDOUT)
• An expression evaluated in scalar context that returns something that can be used as a file handle.

say, print and printf are notable exceptions. They require one of the following:

• A bareword naming a glob (e.g. STDOUT)
• A simple scalar that contains something that can be used as a file handle (e.g. $fh).
• A block that evaluates to something that can be used as a file handle (e.g. { $fhs{$id} }).

A bareword may be provided to subs using the «*» prototype (passing the bareword as a string).

Update: What can be used as a handle vs what syntax can be used to provide it was poorly distinguished originally.