The problem is that you're not doing the search correctly.

You wrote:

s#<--/--a8300-->.*?<--/a8300-->.##isg $_;
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but the correct syntax is just

s#<--/--a8300-->.*?<--/a8300-->.##isg;
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without the trailing ' $_' - the syntax of s/// is actually

$string=~s/from/to/opts;
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with the $string being acted on defaulting to $_. Putting $_ after the expression makes no sense to perl...