No, smart match operator cannot do this. I thought, maybe, you could use the return value, but no, you can't, the return value of smart match operator is boolean (true/false).

My test code

$ perl -E " my( $r, $a, @bba ) = ( 0, 1, 2..4,1,2..4 );  $r = $a ~~ @b
+ba; say $r; "
1
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You could accomplish this task by way of overload

#!/usr/bin/perl --
use 5.010;
use strict;
use warnings;
my @array = ( 2..4,6,2..4 );
say "@array";
my $r = 6 ~~ bless\@array,'smart::ix';
say "$r $$r = $array[$$r]";

BEGIN {
    package smart::ix;
    use overload '~~' => \&rix;
    sub rix {
        my( $self, $val, $selfOnRight ) = @_;
        my $ix = 0;
        while( $ix <= @$self ){
            return  \$ix if $val ~~ $self->[$ix];
            $ix++;
        }
        return ;
    }
    $INC{'smart/ix.pm'} = __FILE__;
    1;
}
__END__
2 3 4 6 2 3 4
SCALAR(0x9b386c) 3 = 6
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This could make a good feature, say use feature 'smarter'; makes ~~ return an object, and sets the global variable $s (for smart) to the same value (like sort does for $a and $b), and ->ix returns the index (for this use case), or ->matches for the general case ....

Hmm, now that I think about, after consulting http://search.cpan.org/~jesse/perl-5.14.1/pod/perlsyn.pod#Smart_matching_in_detail some more, ~~ seems rather limited , oh well