replacing the last portions of an octet

[diamondsandperls]

I need to subtract one digit from the last octet of the first ip and i need to add one to the last octet of the second ip address.

I am obviously struggling with this as my current code will dictate.

SAMPLE STRING: 192.168.1.1-192.168.1.254 needs to be 192.168.1.0-192.168.1.255

I have many strings of text that are random but the format is similar but the sample string can be 172.15.29.21-172.15.30.23 needs to be 172.15.29.20-172.15.30.24. These are all made up. In the first octet there will never be a zero as the last digit so only the very last digit in the octet needs to be regexed and subracted.

#!C:/Perl/bin/perl.exe

use strict;
use warnings;

my $ip_1 =~ m/(\d)-/g;
    $ip_1 = int($ip_1);

my $ip_2 = $ip_1 - 1;

my $input_file = 'CrystalReportViewer1.csv';
my $output_file = 'CrystalReportViewer2.csv';

    open(my $output_fh, '>', $output_file)
        or die "Failed to open $output_file - $!";
        
    open(my $input_fh, '<', $input_file)
        or die "Failed to open $input_file: $!";

    while ( <$input_fh> ) {
            s/$ip_1/$ip_2/;
            
            
            print {$output_fh} $_;
        }
    
    close $output_fh;