Anonymous is correct, you could use those modules, in the interest of sticking with very basic code...

#!/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 an
+d line endings have been removed(\n or \r\n) 
close IN;    #so you don't need to hold the file open anymore, close i
+t

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...
}
[download]

I did not test this time, so you may find an typo or two, but am sure you can handle that with warnings turned on and all...

If you are just learning Perl, you might want to fire this up using 'perl -d (whatever you name this script). The debugger (-d) is a nice way to step through and find out how things work. You can find info in the debugger here, in the pods, or on the web...

enjoy