Don't use select, just write/read to the filehandle you want

#!/usr/bin/perl
use strict;
use warnings;
use HTML::TreeBuilder;

my $filename = 'result.txt';
open OUT, '>', $filename 
  or die "cannot open $filename for writing ; $!";

my $root = HTML::TreeBuilder->new;
$root->parse_file('file2.html');

my @div_class = $root->find_by_attribute("class","forminput");
foreach my $node (@div_class) {
   print OUT $node->content_list();
   print OUT  "\n";
   #$node->dump; # use for debug
}
close OUT;

open IN, '<', $filename 
  or die "cannot open $filename for reading ; $!";
print "\nReading from $filename\n";
while (<IN>){
  print $_;
}
close IN;
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