use CGI;
use CGI::Taint;

my $q=CGI->new();

my $tainted_var = $q->param('form_input');

# first case - dies to browser with 
# "attempt to print tainted var at line 10"
print $q->header.
      "Tainted: $tainted_var";
exit(0);

# second case - no error
my $untainted_var = '';
$tainted_var =~ /(\w\s+)/ and $untainted_var=$1;
print $q->header.
      "Untainted: $untainted_var";
exit(0);