perl -wMstrict -le
"my $c = qr{ (?> -- [^\n]* (?:\n|\z)) }xms;
 my $str = qq(a --b x\n x);
 print qq(match at positions $-[1]-), $+[1]-1
   if $str =~ m{ ($c* [ ] x) }xms;
"
match at positions 2-9

perl -wMstrict -le
"my $c = qr{ (?> -- [^\n]* (?:\n|\z)) }xms;
 my $str = qq(a --b x\n x);
 print qq(match at positions $-[1]-), $+[1]-1
   if $str =~ m{ ($c*  x) }xms;
"
match at positions 6-6

perl -wMstrict -le
"my $c = qr{ (?> -- [^\n]* (?:\n|\z)) }xms;
 my $str = qq(a --b x\n x);
 print qq(match at positions $-[1]-), $+[1]-1
   if $str =~ m{ ($c* ^ [ ] x) }xms;
"
match at positions 2-9

perl -wMstrict -le
"my $c = qr{ (?> -- [^\n]* (?:\n|\z)) }xms;
 my $str = qq(a --b x\n x);
 print qq(match at positions $-[1]-), $+[1]-1
   if $str =~ m{ ($c* ^ x) }xms;
"