use warnings;
use strict;

my $DATAFILE = 'data.txt';
my $INSERTFILE = 'insert.txt';
my $OUTFILE = 'output.txt';

open my $ofh, '>', $OUTFILE or die "$OUTFILE: $!";
select($ofh); # "print"s will go to this handle now

open my $dfh, '<', $DATAFILE or die "$DATAFILE: $!";
my $state = 'before nodes';
while ( my $line = <$dfh> ) {
	if ( $line =~ /^\s*NODE\b/ ) {
		if ( $state eq 'before nodes' )
			{ $state = 'in nodes'; }
		elsif ( $state eq 'in nodes' ) { }
		elsif ( $state eq 'after nodes' )
			{ die "expected only one section of NODEs" }
		else { die $state }
	}
	else {
		if ( $state eq 'before nodes' ) { }
		elsif ( $state eq 'in nodes' ) {
			do_node_insert($INSERTFILE);
			$state = 'after nodes';
		}
		elsif ( $state eq 'after nodes' ) { }
		else { die $state }
	}
	print $line;
}
close $dfh;

sub do_node_insert {
	my $file = shift;
	open my $ifh, '<', $file or die "$file: $!";
	while ( my $line = <$ifh> ) {
		next unless $line =~ /^\s*NODE\b/;
		print $line;
	}
	close $ifh;
}