If you want to do arithmetic on IPv4 addresses, it's easier to convert them into numbers first.

my $ip_num = unpack 'N', pack 'C4', split /\./, $ip_dotted;
$ip_num = ( ( $ip_num - 1 ) ^ 1 ) + 1;
$ip_dotted = join '.', unpack 'C4', pack 'N', $ip_num;
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Note that inet_aton $ip_dotted (from Socket) can be used instead of pack 'C4', split /\./, $ip_dotted.
Note that inet_ntoa $ip_packed (from Socket) can be used instead of join '.', unpack 'C4', $ip_packed.