shenme has asked for the wisdom of the Perl Monks concerning the following question:

I came across a strange side effect while using File::Spec::tmpdir. $ENV{TMPDIR} is being created and set to undef, as though it is being autovivified. I pulled the pieces out of File::Spec::Win32 and ::Unix and got it down to this example:
#!perl -w use strict; use warnings; sub see_args { printf "I see args '%s'\n", join("', '",@_); return; } warn sprintf "In test, at start (1): TMPDIR '%s'\n", ! exists $ENV{TMPDIR} ? '<absent>' : ! defined $ENV{TMPDIR} ? + '<undef>' : $ENV{TMPDIR}; see_args( @ENV{qw(TMPDIR TEMP TMP)} ); warn sprintf "In test, at end (2): TMPDIR '%s'\n", ! exists $ENV{TMPDIR} ? '<absent>' : ! defined $ENV{TMPDIR} ? + '<undef>' : $ENV{TMPDIR};
produces output:
In test, at start (1): TMPDIR '<absent>' Use of uninitialized value in join or string at line 7. I see args '', 'C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp', 'C:\DOCUME~1\ADMI +NI~1\LOCALS~1\Temp' In test, at end (2): TMPDIR '<undef>'

Just to be sure it wasn't the _use_ of the subroutine arguments, I commented out the printf, and got output

In test, at start (1): TMPDIR '<absent>' In test, at end (2): TMPDIR '<undef>'
so it is the call itself doing the nasty thing to my mind. Can someone make this make sense to me?