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$` is the same as substr($var, 0, $-[0]) 
$& is the same as substr($var, $-[0], $+[0] - $-[0]) 
...
$1 is the same as substr($var, $-[1], $+[1] - $-[1]) 
$2 is the same as substr($var, $-[2], $+[2] - $-[2]) 
$3 is the same as substr $var, $-[3], $+[3] - $-[3])

my $q = <<End_Text;
some_letters-01-some_other_letters
...
    print "Mask start index is ", $start_index, "\n";
}