There are two modules which I think you should have a look at - Net::CIDR and more importantly, Net::Netmask. The latter of these modules incorporates a number of functions for exactly what you are requiring. eg.
 

use Net::Netmask;

my ($block) = new Net::Netmask ("192.168.1.5", "255.255.255.0");
print $block->base(), "/", $block->bits(), "\n";
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Ooohhh, Rob no beer function well without!