#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11108019
use warnings;

my %processed;

while( 1 )
  {
  my %files = map +($_ => 1), glob '/root/fd/fg/*.txt';

  for my $file ( grep !$processed{$_}, keys %files )
    {
    # open and process $file here
    }
  %processed = %files; # clean up %processed

  sleep 10; # or some useful sleep time
  }