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my $from_outside=<STDIN>; my $from_inside='Hey!';
tainted($from_outside); #true
tainted($from_inside); #false
tainted($ENV{PATH}); #true

tainted(($untainted1+$untainted2).$tainted); #true
tainted(($untainted1+$untainted2).$untainted3); #false

open(FH, $tainted);
system($tainted);
`$tainted`;
eval($tainted);
exec($tainted);

if($file_name=~/^(\w+)$/) {
    $file_name=$1;  #no longer tainted
}

open(FH, "> users/grades/$file_name") or die "Can't open grades for $f
+ile_name: $!";