...

  my $attach = "file.png";
 
 
  my @fh = split(/\\/, $attach);
  
  my $last_elmnt = @fh;
  my $filename = $fh[$last_elmnt-1];


  my $tempfile = "tempdir/$filename";


  my $new_filename = "allfiles.png";

  open( ATTA, ">$tempfile" ) || die "Can't open $tempfile: $!";
  binmode ATTA;


  while ( <$attach> ) {

    rename $tempfile, "tempdir/".$new_filename or die "Cannot rename file: $!"; }

    print ATTA;

  }
  close ATTA;

...