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my $str = "TXXXABCDGXXXCCCDTGYYYCCCYYYCC";
local our @matches;
...
   (?!)                       # Try again
/x;  
print "$_\n" foreach @matches;

XXXABCDGXXXCCCDTGYYYCCCYYY
XXXABCDGXXXCCCDTGYYY
XXXCCCDTGYYYCCCYYY
XXXCCCDTGYYY