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};
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produces output:

In test, at start (1): TMPDIR '<absent>'
Use of uninitialized value in join or string at test3.pl 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>'
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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>'
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so it is the call itself doing the nasty thing to my mind. Can someone make this make sense to me?