UPDATE, I reaped this against social norm. I had mentioned that I like extended regexes (/x) so that I can comment them. E.g.

m/^A.TL5
   ([EF]) # my match here
   00/x
[download]

I like this because later the comment will help me to remember what I was doing, and in general regexes can be hard to read. I mistakenly extended this to to s///.

s/^A.TL5
   ([EF]) # my match here
   00
   /
   NEW
   \s+
   {$1}   # replae here
   /x
[download]

Toolic pointed out that extended regexes in s/// only work on the matching part, not the replacing part. So this, I hope, is the legal one:

s/^A.TL5
   ([EF]) # my match here
   00
   /NEW {$1}/x
[download]