Actually you have to use \1 in the left part,
you can't use $1, but you can use either \1 or $1 in
the right part.
$1... cannot be used in the left part because it would
be expanded (the $1 from the previous regexp) and usage
of \1 in the right part is mildly frowned upon, mostly
for stylistic reasons, as it is close to the \nn or \nnn
notation for octal characters (although as usual Perl
will DWIM by interpreting \12 as $12 if there were more
than 12 captured expressions and as the character \12
otherwise).
