...where the parentheses are numbered by the position of the left parentheses, so "abc" =~ /((a)b(c))/ will set $1 to "abc", $2 to "a", and $3 to "c".

If you use a regex that has parentheses in list context, the substrings are returned in a list, and can be used directly:

my ($abc, $a, $c) = "abc" =~ /((a)b(c))/
[download]

see perlop for more information about m// and s/// and what they return and how flags like //g affect them.