In this case, 

s{CO}{C/O}g

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would do. If there's a chance of overlap between the two patterns to separate (e.g. putting "/" between "AC" and "CA"), you need a lookahead. 

s{AC(?=CA)}{AC/}g

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or 

s{AC\K(?=CA)}{/}g

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In reply to Re: How to insert a "/" within a string? by ikegami
in thread How to insert a "/" within a string? by rsvp1920

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