Here's how I do it using threads:

#! perl -slw
use strict;
use threads;
use threads::shared;
use Thread::Queue;

my $Qin = new Thread::Queue;
my $done :shared = 0;

async{
    open my $fh1, '<', "firewall.log"
        or die "FWLog : $!";

    ( local $_ = <$fh1> and $Qin->enqueue( "F1:$_") )
            or Win32::Sleep 10 until $done;
    close $fh1;
};

async{
    open my $fh2, '<', "modem.log"
        or die "Modemlog : $!";
    ( local $_ = <$fh2> and $Qin->enqueue( "F2:$_" ) )
        or Win32::Sleep 10 until $done;
    close $fh2;
};

while( my $input = $Qin->dequeue() ) {
    chomp $input;
    my( $src, $txt ) = split ':', $input, 2;
    if( $src eq 'F1' ) {
        print "Firewall: '$txt'";
    }
    else {
        print "Modem   : '$txt'";
    }
}
[download]

This monitors 2 files, and processes all the data in a single place. It is easy to extend to any number of files and you can process the data within the threads rather than centrally if that fits your needs.