I do something similar to this quite often, and I like to pull the information out of a routing table dump and insert into a Net::Patricia instance.

use Net::Patricia;

my $pt = new Net::Patricia;

while (<>) {
  if ( /((?:\d{1,3}\.){3}\d{1,3}\/\d{1,2})/ ) {
    $pt->add_string($1, 'Inside networks');
  }
}

print "$host: Inside\n" if $pt->match_string($host);
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It saves the trouble of worrying about bitwise operations in string or numeric context and makes for a simple way to store and retrieve CIDR block information.