Your result only depend on the number of words before your substring and the number of words in the substring:

use strict;
use warnings;

my $string = 'this is a test';
my $substr = 'is a test';

if( $string =~ /(.*)\b$substr\b/ ) {      # do we have a match?
    my $n = () = $1 =~ /\b(\w+)\b/g;      # number of words before mat
+ch
    my $m = () = $substr =~ /\b(\w+)\b/g; # number of words in match
    print join(' ',$n+1..$n+$m),"\n";
}
[download]