Just to add to all the posters above, who focus more on
untainting the data, perldoc perlsec gives this example
to beforehand check if the data is actually tainted:

sub is_tainted {
 return ! eval {
  join('',@_), kill 0;
  1;
 };
}
[download]

GreetZ!,

ChOas

print "profeth still\n" if /bird|devil/;