Also $1 is not initialised on the LHS of the regex whereas \1 is...

use warnings;
$_ = 'oooooo';
s/(o)$1/foo/;
s/(o)\1/foo/;
s/(o)/\1/;
s/(o)/$1/;

__END__
Use of uninitialized value in concatenation (.) at script line 3.
\1 better written as $1 at script line 5.
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print