use strict;
use warnings;
use IO::File;
use DNS::ZoneParse;

my @zonefiles = </path/to/zonefile/*>;

foreach my $zonefile (@zonefiles) {
    my $z = DNS::ZoneParse->new($zonefile);

    # change ttl in all MX records, for example
    my $mx = $z->mx;
    $_->{ttl} = 'foo bar' for (@$mx);  # update ttl

    $z->new_serial();

    # write the new zone file to disk
    my $zf = new IO::File "$zonefile", "w"
        or die "Error writing zonefile: $!";
    print $zf $z->output();
}