The special variable @- is an array that contains the offsets from the start of the string to each match. Consider:

use strict;
use warnings;

for my $str ('1 2 3', 'a b c', ' a3 b2 c1') {
    next if $str !~ /\w?([0-9])/;
    my $mpos = $-[0];
    my $cpos = $-[1];
    
    print "Found $1 in '$str' at index $cpos. Overall match started at
+ $mpos\n";
}
[download]

$-[0] accesses the first element of the array - the start of the entire match. $-[1] gives the index of the start of the first capture. The script prints:

Found 1 in '1 2 3' at index 0. Overall match started at 0
Found 3 in ' a3 b2 c1' at index 2. Overall match started at 1
[download]

See the perlvar regular expression variables section. You should take a look at perlretut and perlre too.