my $value;
while (condition) # or for loop
{
  $value <op>= something;
}

##</code><code>##

my $sum; 
$sum += $_ for @nums;

##</code><code>##

my $valid;
while (<DATA>)
{
  chomp;
  next if /INVALID/;
  $valid .= $_;
}


##</code><code>##

my $product;
$product *= $_ for @nums;