If you have < ~ 2M IP's to check, you could use this (untested):

my %ips_to_check = map {chomp; $_ => undef} <DATA>;

while (my $confLine = <$in>) {
   if ($confLine =~ /ip4-address address="([\d\.]+)"/ 
         and exists $ips_to_check{$1}){
      $confLine =~ s{/>}{update"/>};
    }
    print {$out} $confLine;
}
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