... very naive assumption ...

To call a feature/tool which you don't understand "very naive assumption" , I think is , very naive, ridiculously so :)

Anyway, re#'taint' mode

$ perl -MTaint::Util -Mre=taint -Tle " $_ = $ENV{PATH}; sub f{printf q
+q/\$_ tainted? %d\n/, tainted $_;} f; $_ = $1 if /
(.*)/; f; untaint $_; f;"
$_ tainted? 1
$_ tainted? 1
$_ tainted? 0
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