Of course, you need to consider what happens if you have a conflict such as what would happen if you had 2 files called 'test file' & 'test_file' - how would you handle this?

If it were me, I would build a hash of arrays, the key of the hash being the new file name, and the array elements being all files that would have that new name. Here's a quicky:

#!perl -w

use strict;

opendir(DIR,".") or die "Couldn't opendir on .: $!";

while($file = readdir(DIR)){
    my $oldfile = $file;
    $file =~ s/\s+/_/g;
    push @{$hash{$file}}, $oldfile;
}
closedir(DIR) or warn "Couldn't close directory .: $!";
foreach my $key (keys %hash){
    if(scalar(@{$hash{$key}}) != 1){
        print "The following files would have been renamed to the same
+ thing:\n"
        local $"="\n";
        print "@{$hash{$key}}", "\n";
    }
    else{
        print "$hash{$key}[0] -> $key "
        my $status = rename $hash{$key}[0], $key;
        if(!$status){
            print "Failed\n";
        }
        else{
            print "Succeeded\n";
        }
    }
}
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thor