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use strict;
use warnings;
...
    warn sprintf "In test, at start (1): TMPDIR '%s'\n",
        ! exists $ENV{TMPDIR} ?  '<absent>' : ! defined $ENV{TMPDIR} ?
+ '<undef>' : $ENV{TMPDIR};
print Dumper(@ENV{qw(TMPDIR TEMP TMP)});

$VAR1 = undef;
$VAR2 = 'C:\\DOCUME~1\\someone\\LOCALS~1\\Temp';
$VAR3 = 'C:\\DOCUME~1\\someone\\LOCALS~1\\Temp';