You would need to split the string according to what surrounds a word, im assuming in this case it is a space:

@words = split ' ', $string;
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The third word would be $words[2] so you'd create the string again without that word:

for($i=0;$i<scalar(@words);$i++){
$string .= $words[$i] unless $i==2;
}
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Keep the array of words, and when you need to put it back in simply do:

for($i=0;$i<scalar(@words);$i++){
$string .= $words[$i];
}
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Hope this helps

Cheers
Sam