... $-[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'
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(And BTW: I've been practicing goofs for quite a while myself.)