use Text::Diff qw(diff);
[...]
$diff = diff "$config_dir/$config_old", "$config_dir/$config_new", 
        { STYLE => "OldStyle"};
@diff = split '\n', $diff;
[...]

##</code><code>##

LINE:
for my $line (@diff) {
    next LINE if $line =~ /set password ENC/;
    [...]
}

##</code><code>##

        set private-key "-----BEGIN ENCRYPTED PRIVATE KEY-----
MIIFDjBABgkqhkiG9w0BBQ0wMzAbBgkqhkiG9w0BBQwwDgQInXCep+2zzpgCAggA
MBQGCCqGSIb3DHMHBAiSZZZ3CUL1cQSCBNhxHiU0wI3XOMU05aVZybU6OOJOJBa/
M+b28ad6P8VZiN+eToUfs3pTg+VqzAc273fdnZPZFMClXpJk8kQZv0ruEoA99RqE
pgsnYGVxzZNmDy5HT3yBDGjRCssDnQ8QUBqabFCpW6d7fzilw9PnoHjFRmLxKnNE
[...]

##</code><code>##

    next LINE if $line =~ /{looks like random stuff to me}/;