my $q = <<End_Text;
some_letters-01-some_other_letters
some_other_letters-03-some_other_letters
End_Text

while ( $q =~ m{\w+ \-(\d\d)\- \w* }gx ) {
    my $start_index = $-[1];
    print "Mask start index is ", $start_index, "\n";
}