From perldoc -f open: 
As a special case the 3 arg form with a read/write mode and the
third argument being "undef":

    open(TMP, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file.  Also using
"+<" works for symmetry, but you really should consider writing
something to the temporary file first.  You will need to seek()
to do the reading.

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If FILEHANDLE is an undefined scalar variable (or array or hash
element) the variable is assigned a reference to a new anony-
mous filehandle,

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 In any case IMHO you should either do some checks or rely on Perl's automagic ARGV.

Update: I realized now that you were talking about somebody else's code and you were concerned about the lack of checks yourself in the first place. Also, I removed the unrelevant portions of the quote.

Update2: I knew the answer but in a hurry I quoted the wrong portion of perldoc -f open. There's a lesson in this: never rely too much on searching for 'anonymous'...

In reply to Re: 3-arg open() does not give warnings!? by blazar
in thread 3-arg open() does not give warnings!? by graff

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