$dir = $ARGV[0]; # get dir from arg list
opendir(D, $dir) or die $!;
for(readdir(D)) {
  $file = $_;
  tr/ /_/; # or tr/ \t\r\n/_/; for tabs and spaces
  # or s/\s+/_/g; to make multiple whitespace chars into one _
  rename("$dir/$file", "$dir/$_");
}
closedir(D);