Make scalar tainted

[Артём Варнайский]

 Hello again! 
Prompt me please elegant and 'vanilla' way to taint some scalar. My vars lost 
their taint when I just split external string with regexp on param/val pairs, 
without checking them for correctness.
And What do you say about this:
$foo.=substr($ENV{PATH},0,0); #$foo tainted if $ENV{PATH} is tainted
Thank, and sorry for my runglish :)