this is a simple self-reproducing program

$_=q(s%(.*)%$_=qq(\$_=q($1),$1),print%e),s%(.*)%$_=qq(\$_=q($1),$1),print%e

the program can be split into 2 parts:

variable assignment:   $_=q(s%(.*)%$_=qq(\$_=q($1),$1),print%e)
and printing part:     s%(.*)%$_=qq(\$_=q($1),$1),print%e

the variable assignment parts assigns a string that is basically
code for the printing part (recursion theorem).

the printing part matches our string stored in $_ and prints
it out twice (except that it adds $_='' around the first string).
