This is what I tried based on your question. So, I specified a new delimiter, in the case, the '}', and then I just check if 'service_description' is present or not, and so on.

Let me know if I'm missing something.

#!/usr/bin/perl

use strict;

local $/ = '}';
while( <DATA> ) {
    if ( /\s+service_description\s+/ ) {
        next;
    }
    else {
        s/(use\s+)sam(\w+)/$1$2/;
    }
    print;
}

__DATA__
define service{

        use                             sam_win_cpu_load

        host_name                       buo.va.com

        contact_groups                  buo_admins

}
define service{

        use                             generic-service

        host_name                       buo.va.com

        service_description             MS SQL Server

        check_command                   check_service_state!MSSQLSERVE
+R!public

        contact_groups            buo_admins

}
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