How about something like:

#!/usr/bin/perl
use strict;

# @ARGV is = to @_ by default
my $path=shift or die "Usage: $0 <directory|file>\n";

# Is it a directory or a file?
if (-d $path) {
  opendir(DIR,$path) or die "could not open $path: $!\n";
  for my $file (readdir(DIR)) {
    if ($file =~ /\_/) {
      nounder($file);
    }
  }
  closedir(DIR);
}
else {
  nounder($path);
}

sub nounder {
  my $file=shift;
  my $newfile=$file;
  $newfile=~s/\_/ /g;
  rename($file,$newfile) or die "couldn't rename $file: $!\n";
  print "renamed $file to $newfile\n";
}
[download]

-vladdrak