There are network-address functions...

use Socket;

my $n32 = inet_aton("192.168.254.0");
print inet_ntoa(pack("N",unpack("N",$n32)-1));  # "192.168.253.255"
[download]

Or, in the OP's context:

s/(\d+\.\d+\.\d+\.\d+)/inet_ntoa(pack("N",unpack("N",inet_aton($1))-1)
+)/e;
[download]

But (to quote the docs): "For portability do not assume that the result of inet_aton() is 32 bits wide, in other words, that it would contain only the IPv4 address in network order."