That would explain $$, but it seems to happen with all variables..

#!/usr/bin/perl -T
use strict;
use warnings;

$ENV{PATH} = '/usr/bin:/usr/local/bin:/bin';
$ENV{ENV} = '';

my $untainted = 'foo';
my $temp_fails = [ 
                   `mktemp /tmp/temp.$untainted.XXXXXX`,  
                   `mktemp /tmp/temp.$untainted.XXXXXX`,
                 ];
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