Like tommyw and scain said, you have to escape the backslash. Also, I don't think \d matches hex digits 'A'-'F'. If you need to match those (looks like you do from the sprintf), you can use [[:xdigit:]] instead. 
s/\\x([[:xdigit:]]+)/'&#'.hex($1).';'/eg
Btw, if you only want 2 digits (so that stuff like "\x92Efficiency" doesn't confuse the regex), use {2} instead of +. If you want to match one or two, use {1,2}. You're probably better off matching an exact number rather than a range though if you can. 
s/\\x([[:xdigit:]]{2})/'&#'.hex($1).';'/eg
Hope this helps!

Update: Oops, I just read your reply to tommyw above. All you need is a range, combined with ord (not hex). 

s/([\x80-\xFF])/'&#'.ord($1).';'/eg

In reply to Re: Control Characters (\xNN) in HTML by blackmateria
in thread Control Characters (\xNN) in HTML by garliqua

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