Use a regex that encompasses the entire first two:
This is an untested regex. It also assumes that the first two are always in the exact format from above. You may need to change the specifics, but this is the general idea 
s/^\d_(\w+)\/\d_(\w+)/$1\/$2/;

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When's the last time you used duct tape on a duct? --Larry Wall

In reply to Re: changing only the first two matches of a regex by ColonelPanic
in thread changing only the first two matches of a regex by Anonymous Monk

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