use File::Find;

    finddepth sub { # finddepth so directories are done AFTER their contents
      my $old = $_;
      tr/ /_/ or return; # don't do anything if you don't have spaces
      -e and return; # don't rename over an existing file!
      rename $old, $_ or warn "cannot rename $old to $_: $!";
    }, "/top/dir/here";