{
   no warnings 'substr';
   ... use substr here ...
}

##</code><code>##

my $substr;
if (length($_) >= 4) {
   $substr = substr($_, 4, 3);
} else {
   $substr = '';
}

##</code><code>##

sub find_process {
   ...
   if (length($file) < $x) {
      warn("Skipping file $file\n");
      return;
   }
   ... substr($file, $x, ...) ...
   ...
}

##</code><code>##

sub find_process {
   no warnings 'substr';

   ...
   my $part = substr($file, $x, ...);
   if (not length $part) {
      warn("Skipping file $file\n");
      return;
   }
   ...
}