#!/usr/bin/perl
use strict;
use warnings;
my $file = "input.txt";

open(IN, "<", $file) or die (Can't open $file, $!\n");
my @in = <IN>;
chomp @in;   #all records(lines) from the file are now in array @in and line endings have been removed(\n or \r\n) 
close IN;    #so you don't need to hold the file open anymore, close it

while (my $line = @in){          #you could do a foreach here too, the rest of this should be familiar to you
	if ($line =~ m/in-addr/) {
		my @line = split(/\./, $line);
		if ($line[2] =~m/in-addr/) {
			process_line("$line[1]\." ."$line[0]\." . "0\." . "0"  . "\/24");
		} else {
			process_line("$line[2]\." . "$line[1]\." ."$line[0]\." . "0" . "\/16" );
		}
	} else {
		process_line($line);
	}
}

sub process_line {
	print shift . "\n";
	# do whatever...
}