From perldoc perlre:

x   Extend your pattern's legibility by permitting
           whitespace and comments.
...
...
The /x modifier itself needs a little 
more explanation. It tells the regular 
expression parser to ignore
whitespace that is not backslashed or 
within a character class. You can use 
this modifier to break
up your regular expression into (slightly) 
more readable parts.
[download]

It *can* be inferred from here that /x applies only to the first part of s///, but it's not really conclusive. Perhaps this can be clarified with a brief note.