use Net::DNS;
use Net::DNS::ZoneFile;

my $rrs = Net::DNS::ZoneFile->read("/var/named/named.local",
				   { 
				       # You might need to set the
				       # $ORIGIN explicitly here if
				       # your zone file does not make
				       # this explicit. See perldoc
				   }
);

# Print the RRs that were fetched
print $_->string, "\n" for @$rrs;

# Add a sample RR
push @$rrs, new Net::DNS::RR "3 7200 IN PTR always-listed.your.domain.";

# Update the serial :)
# You can also "replace" the current serial, with the current date,
# as commonly recommended. However, if you're looking into this, you
# probably have better change control on your zones already.
foreach (@$rrs)
{
    next unless $_->type eq 'SOA' and $_->class eq 'IN';
    $_->serial($_->serial + 1);
}

# Print the resulting RR set (this could be your new zone by printing to
# a file)
print $_->string, "\n" for @$rrs;