I think the following little script should be enough to show you how to proceed:

#! perl
use strict;
use warnings;
use diagnostics;
use Data::Dumper;

my $destination = '92.123.72.112';
my $final_ip    = '92.123.72.0';

# Get the first 3 octets of each IP

my $destn_prefix = $destination =~ s/ \. \d{1,3} $ //rx;
my $final_prefix = $final_ip    =~ s/ \. \d{1,3} $ //rx;

# Populate the hash

my %hash;

if ($destn_prefix eq $final_prefix)
{
    $hash{$destn_prefix} = [ $destination, $final_ip ];
}

print Dumper(\%hash), "\n";

# Access a given location

my $ip = '92.123.72';

if (exists $hash{$ip})
{
    my @ips = @{ $hash{$ip} };

    print 'IPs at location ', $ip, ' are: ', join(', ', @ips), "\n";
}
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Output:

$VAR1 = {
          '92.123.72' => [
                           '92.123.72.112',
                           '92.123.72.0'
                         ]
        };

IPs at location 92.123.72 are: 92.123.72.112, 92.123.72.0
[download]

Some useful references:

• The substitution operator: Regexp Quote Like Operators
• References: perlreftut
• Dereferencing: Mini-Tutorial: Dereferencing Syntax

Hope that helps,