How about:

$string = "one two three two four";
$gnirts = reverse $string;
$gnirts =~ s/ owt//;
$string = reverse $gnirts;
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Reverse the string, remove the reverse of the desired token, then reverse the result.

Update: Full disclosure -- this was motivated by a node on sexeger I saw before. Props to japhy.

Update2: Fixed typo -- thanks Hofmator.