foreach my $n (@natures){
open (OUT, "> $n.txt") or die "blah blah $! \n"
while(<FILE>){
 print OUT $_ if $_ =~ /$n/;
}
}

##</code><code>##

while(<FILE>){
foreach my $n (@natures){
if($_ =~ /$n/){
open (OUT, ">> $n.txt") or die "blah blah $! \n"
print OUT $n;
close OUT;
}
}
}