Parse the config file and change the values in the resulting data structure, let's say we want to default the username on the servers:

use strict;
use warnings;
use Data::Dumper;
use Config::General;

my $conf = Config::General->new( -ConfigFile => \*DATA );
my %config = $conf->getall;

$_->{username} ||= 'Pascal2020' for $config{server1}, $config{server2}
+;

print Dumper \%config;


__DATA__
<server1>
    host                = server1.domain.tld
    ssl                 = true
    port                = 443
    username             =
    password             = qwerty
    vhost                = example
</server1>
<server2>
    host                = server2.domain.tld
    ssl                 = false
    port                = 2080
    username             =
    password             = qwerty
    vhost                = example
</server2>
[download]

There are many ways to do this, what did you try?

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)