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.

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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).

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

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