while (<DATA>) {
    next if ! /^DATA/;
    # ...

##</code><code>##

<DATA> for 1 .. 3;
while (<DATA>) {
    # ...

##</code><code>##

my $found_start_of_data;
while (<DATA>) {
    last if $found_start_of_data;
    # ... some complex code that sets the flag
}
while (<DATA>) {
    # ...
}

##</code><code>##

# Extract
my ($want) = $data =~ m{(some_regular_expression)};

# Eliminate
$data =~ s{some_regular_expression}{};