This may be what you want:

$ perl -le'
my %hashIPs = qw(
    192.168.1.0  1
    192.168.1.1  1
    192.168.1.2  1
    192.168.1.4  1
    192.168.1.5  1
    192.168.1.6  1
    192.168.1.10  1
    192.168.1.11  1
    192.168.1.12  1
    );

my %check;
for my $ip ( keys %hashIPs ) {
    next unless $ip =~ /^(\d+\.\d+\.\d+)\.(\d+)$/;
    $check{ $1 } = 0 x 256 unless exists $check{ $1 };
    substr $check{ $1 }, $2, 1, 1;
    }

for my $key ( keys %check ) {
    print "Missing: $key.$-[0] - $key.", $+[0] - 1, "\n"
        while $check{ $key } =~ /(?<=1)0+(?=1)|^0+|0+$/g;
    }

'
Missing: 192.168.1.3 - 192.168.1.3
Missing: 192.168.1.7 - 192.168.1.9
Missing: 192.168.1.13 - 192.168.1.255
[download]

Instead of using grep:

my @segments = grep {/^(\d{1,3}\.\d{1,3}\.\d{1,3})/} keys %hashIPs;
[download]

You need to use map:

my @segments = map /^(\d{1,3}\.\d{1,3}\.\d{1,3})/, keys %hashIPs;
[download]