if ($old) {
  ## deprecated oids
  $wrnet        = ".1.3.6.1.4.1.9.2.1.55.0";
  $confnet      = ".1.3.6.1.4.1.9.2.1.53.0";
  $wrmem        = ".1.3.6.1.4.1.9.2.1.54.0";
  $flashtotftp  = ".1.3.6.1.4.1.9.2.10.9.0";
  $tftptoflash  = ".1.3.6.1.4.1.9.2.10.12.0";
  $eraseflash   = ".1.3.6.1.4.1.9.2.10.6.0";

} else {
  ## oids
  $wrnet        = ".1.3.6.1.4.1.9.2.1.55.0";
  $confnet      = ".1.3.6.1.4.1.9.2.1.53.0";
  $wrmem        = ".1.3.6.1.4.1.9.2.1.54.0";
  $flashtotftp  = ".1.3.6.1.4.1.9.2.10.9.0";
  $tftptoflash  = ".1.3.6.1.4.1.9.2.10.12.0";
  $eraseflash   = ".1.3.6.1.4.1.9.2.10.6.0";
}
[download]

These variables that you are binding and using as separate scalars should be encapsulated into two different hashes, one for each set oids:

my $data{old} = {
wrnet        = ".1.3.6.1.4.1.9.2.1.55.0",
  confnet      = ".1.3.6.1.4.1.9.2.1.53.0",
  wrmem        = ".1.3.6.1.4.1.9.2.1.54.0",
  flashtotftp  = ".1.3.6.1.4.1.9.2.10.9.0",
  tftptoflash  = ".1.3.6.1.4.1.9.2.10.12.0",
  eraseflash   = ".1.3.6.1.4.1.9.2.10.6.0"
}

my $data{new} = {
wrnet        = ".1.3.6.1.4.1.9.2.1.55.0",
  confnet      = ".1.3.6.1.4.1.9.2.1.53.0",
  wrmem        = ".1.3.6.1.4.1.9.2.1.54.0",
  flashtotftp  = ".1.3.6.1.4.1.9.2.10.9.0",
  tftptoflash  = ".1.3.6.1.4.1.9.2.10.12.0",
  eraseflash   = ".1.3.6.1.4.1.9.2.10.6.0"
}
[download]

also since the first part of these numbers does not change, you should store that in a variable:

my $prefix = ".1.3.6.1.4.1.9.2";
my $number = "$prefix.10.6.0";
[download]