Maybe you should be more explicit with your requirements. For instance, do you want to keep the order of the (first-) occurrences of each word?

If not, doing a split on whitespace, using each list element as a hash key and flattening the result with keys() will give you the list of unique words. 

my $string = "foo bar ...";
my %L;
map($L{$_}++, split(/\s/, $string));
# bonus: each hash element has the number of occurrences as value.
$string = join(' ', keys(%L));

[download]
HTH!
note: this is untested...

--
Cheers, Joe

In reply to Re: remove duplicates by joe++
in thread remove duplicates by cruelty

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