If you use !$!, only $! get interpolated, not the entire expression. In your case, $!, in string context, will be the empty string. So, you end up with 
/^.{!}(.{5})/

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which is not going to match. If you want to interpolate the expression, use: 
/^.{@{[!$!]}}(.{5})/

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In reply to Re: weird regex question by JavaFan
in thread weird regex question by Anonymous Monk

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