51 characters, though I'm sure there's a golf trick that could drive that down further. 

# 345678 1 2345678 2 2345678 3 2345678 4 2345678 5 
s/<!--(.+?)-->/(my$x=$1)=~s#--#-¬#g;"<!--$x-->"/seg

[download]

Hm... Looks like a nearly identical approach to blockhead's first approach. (Evil minds think alike.) Here's a tweak that takes it to 53 characters. 

# 345678 1 2345678 2 2345678 3 2345678 4 2345678 5 23
s/<!--(.+?)-->/(my$x=$1)=~s#--#-¬#g;"<!-- $x -->"/seg

[download]

Changed once more (to 49), since I was a doofus and didn't read tye's instructions carefully enough. I think this one loses points for changing too much, but wins for making the comments legal both ways. 

# 345678 1 2345678 2 2345678 3 2345678 4 2345678 
s/<!--(.+?)-->/(my$x=$1)=~s#-#¬#g;"<!--$x-->"/seg

[download]

In reply to Re: Golf: Fix de facto HTML comments by dws
in thread Golf: Fix de facto HTML comments by tye

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