Net::CIDR::Lite would not be good for this. With the caveats moritz mentions above, I'd use a hash:

use strict;
use warnings;

my @ips = qw(
 1.1.1.1 1.1.1.2
 4.2.4.5 4.2.4.6
);

# Keep this many blocks
my $n = 3;
my $re = join "\\.", ("\\d+") x $n;
$re = qr/^($re)/;

my %collapsed;
for (@ips) {
  if ( /$re/ ) {
    $collapsed{$1}++;
  } else {
    warn "Bad ip [$_] or # of blocks [$n]\n";
  }
}

my $zero = join ".", ("0") x ( 4-$n );
print "$_.$zero\n" for keys %collapsed;
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