Explanation for perl quine

Question

I found this quine recently

$a='$a=%c%s%c;printf($a,39,$a,39,10);%c';printf($a,39,$a,39,10);

And I just can't get my head around it. I've found no explanation on google/SO for this particular one, so I'm hoping that someone could explain to me how that thing works :-)

Answer 1

Look at printf parameters and substitute them by hand,

(39 is single quote, ', and 10 is newline \n when interpreted as %c) so $a which start as

$a=%c%s%c;printf($a,39,$a,39,10);%c

becomes (replaced chars marked below with ^)

$a='%s%c;printf($a,39,$a,39,10);%c
   ^ (first %c replaced)
$a='$a=%c%s%c;printf($a,39,$a,39,10);%c%c;printf($a,39,$a,39,10);%c
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ (%s replaced)
$a='$a=%c%s%c;printf($a,39,$a,39,10);%c';printf($a,39,$a,39,10);%c
                                       ^ (second %c replaced)

and finally

$a='$a=%c%s%c;printf($a,39,$a,39,10);%c';printf($a,39,$a,39,10);\n
                                             (last %c replaced) ^^