How does one mark a variable as tainted?

use Taint ();
Taint::taint($untrustedvalue);
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For example, if you use a regex to parse the key-value fields out of a query string and reverse the CGI encoding, you should mark the resulting data as tainted. (The "use CGI or die" advocates will tell you that you shouldn't be writing your own function for that anyway, but hat's another debate for another thread.)

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
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