That's overly complicated. Move the \b to the end. 
$line =~ /\bh(?:e(?:l(?:l(?:o)?)?)?)?\b/i;

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In 5.10, you can even disable needless backtracking 
$line =~ /\bh(?:e(?:l(?:l(?:o)?+)?+)?+)?+\b/i;

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In reply to Re^2: Regex - Matching prefixes of a word by ikegami
in thread Regex - Matching prefixes of a word by SuicideJunkie

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