#!/usr/bin/perl -w

use strict;

my $line;

open INPUT_FILE, "input_filename" or die "Wonky!\n";
open FIRST_FILE, ">filename1" or die "Hork!\n";
open SECOND_FILE, ">filename2" or die "Foo!\n";
open THIRD_FILE, ">filename3" or die "Spew!\n";

while ($line = <INPUT_FILE>)
{
    if $line =~ /first_criteria/
    {
        print FIRST_FILE $line;
    }
    else if $line =~ /secod_criteria/
    {
        print SECOND_FILE $line;
    }
    # And so on.
}