No explanation (yet), just an observation: strace shows Perl is creating temp files in all but the last case where the file name slot is $yund: 

...
open("/tmp/PerlIO_Ol6dlF", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/tmp/PerlIO_Ol6dlF")            = 0
open("/tmp/PerlIO_WFvCgM", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/tmp/PerlIO_WFvCgM")            = 0
open("/tmp/PerlIO_su06bT", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/tmp/PerlIO_su06bT")            = 0
open("/tmp/PerlIO_HAAg8Z", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/tmp/PerlIO_HAAg8Z")            = 0
Use of uninitialized value in open at ./799021.pl line 32.
open("", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 ENOENT (No such file or 
+directory)
...

[download]

Probably some special handling when the file name is an explicit scalar variable as opposed to just an undefined value...

(This is generally documented: 

    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.

[download]

but it doesn't say anything about what happens if the undef isn't written literally...)

In reply to Re: open undef shift by almut
in thread open undef shift by Anonymous Monk

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