Or, translated into lookbehind notation (and moving the + for more efficiency): 
$string =~ s/(?<=[^Y])X+//g;

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As long as there's a non-Y character preceding, remove any string of Xs.
The PerlMonk tr/// Advocate

In reply to Re: Re: Variable-width negative lookbehind by Roy Johnson
in thread Variable-width negative lookbehind by dragonchild

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