Update: Hrm, I guess I'm a very slow typer today.

Opening and closing files once for every line? Why in the world would you want to to that? Consider the following:

#!/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.
}
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But I have no idea where the @natures array comes from. Is that your criteria for sorting the lines?