#!/usr/bin/perl -w
use strict; 

use File::Find;
use File::Basename;

my $start_dir = "C:/temp"; 
my $log_file = 'C:/temp/logfile.txt';
open (my $log, '>', $log_file) or die "cannot open $log_file for write";

find (\&process_file, $start_dir);

sub process_file
{
    return unless -f;  # do not process directories, 
                       # only "real files"
    my $full_path_name = $File::Find::name;
    print "$full_path_name\n";  
    
    #other ways of writing the "if", but extending your code
    
    if ( -s == 0 and ($_ ne 'empty.txt') and ($_ ne 'sample.txt') and 
                     ($full_path_name ne $log_file) 
       )
    {
       print $log "Warning: $full_path_name has size ZERO\n";
    }    
}