As a variation, the @- 'match start' variable can be used (see perlvar):

perl -wMstrict -le
"my $string = 'foo/bar/pattern/baz/pattern/quux';
 my $pattern = qr{ pat{2}ern }xms;
 if ($string =~ m{ ($pattern) }xms) {
   print qq(start: $-[1]);
   }
 else {
   print 'pattern not found';
   }
 my $idx = index $string , 'pattern';
 print qq(index: $idx);
"
start: 8
index: 8
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Update:

As per the documentation, a hyphen renders as a space in [doc://name#anchor] links. How can I get a hyphen to render as a hyphen? (Thanks toolic — @- looks like @- now.)