... $-[0] requires /...(...).../ re positional variables.
You show a capturing group in your example and refer to "positional variables", so I thought I'd mention that $-[0] is the offset of the start of the overall match and does not depend at all on capture groups.
c:\@Work\Perl>perl -wMstrict -le
"my $s = 'xxxabcyyy';
;;
print qq{overall match begining at offset $-[0] in '$s'}
if $s =~ m{ abc }xms;
"
overall match begining at offset 3 in 'xxxabcyyy'
(And BTW: I've been practicing goofs for quite a while myself.)