What Ikegami says. And there's the "%v" template (vector flag) for sprintf. So, since this is in obfuscation, here's a golfed cidr calculator:

#!/usr/bin/perl
# file cidr.pl
$m=pack B32,pop=~'/'x$';printf"$` network %vd broadcast %vd netmask %v
+d\n",($z=eval$`)&$m,$z|~$m,$m
[download]

Note the eval$` bit, which is essentially what happens in your code. Usage example:

shmem [qwurx] ~ > cidr.pl 172.16.242.150/27
172.16.242.150 network 172.16.242.128 broadcast 172.16.242.159 netmask
+ 255.255.255.224
[download]

This cidr appeared one long gone day on IRC in the perlgolf channel iirc.
I guess it is off the keyboard of tybalt89, but I might be wrong.