It helps to think about what you want to do.

• If you find webserverX, you want to forget that you've seen serverX.
• If you find serverX, you want to ignore it if you've seen webserverX.

Then coding becomes simple.

my %servers;
while (<>) {
   my ($server) = /^([^:]+):/ or next;
   delete($servers{$1}) if $server =~ /^web(.*)/;
   $servers{$server} = 1 if !$servers{"web$server"};
}

my @servers = keys(%servers);
[download]

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