# good, open with error checking
opendir(DIR, "$dir") || die "can't open $dir $!\n";

# maybe name the variable @filenames - as there is more than one
# or get rid of the variable altogether by saying
# foreach my $oldname ( readdir(DIR) )
my @filename = readdir (DIR);

# declare filename with my and give better name (see above)
# foreach my $oldname (@filename)
foreach $filename (@filename) { 

  # this matches against the special variable $_
  # (which you are not using) you want
  # if ($filename =~ / /)
  if (/ /) {  
    
    # the regex is completely wrong you want either
    # s/ +/./g or tr/ /./ try to figure out the difference
    # and $newfile is never assigned a value so:
    my $newfile = $oldfile;
    $newfile =~ s/ +/./g;

    # you don't have to do the rename when you didn't
    # change anything, so
    rename($oldname, $newfile) unless $oldname eq $newfile;
  }
}
closedir(DIR);