I don't quite understand how we get from toniax's OP to finding all starting positions of the substring, but here's another way:

>perl -wMstrict -le
"my $str = qq{foo data } x 5;
 print $str;
 ;;
 print qq{start of 'data' at $-[1]} while $str =~ m{ (data) }xmsg;
"
foo data foo data foo data foo data foo data
start of 'data' at 4
start of 'data' at 13
start of 'data' at 22
start of 'data' at 31
start of 'data' at 40
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See @- (and its cousin @+) in perlvar.