... an article that explains the syntax usage ...
 = $_ for @_;
I'm guessing this is shorthand for a for loop ...

This is the statement modifier form of a for-loop. It iterates over the elements of a  LIST and 'topicalizes' (or aliases with localization) the default scalar  $_ (see perlvar) to each element in turn. Because  $_ is aliased, elements of an array can be altered "in place", which can be very helpful, even essential, in dealing with large arrays.

>perl -wMstrict -le
"$_ = 'same as it ever was';
 ;;
 my @ra = (1, 2, 3, 4);
 ;;
 $_ = $_ + 990 for @ra;
 print qq{@ra};
 ;;
 print 880 + $_ for 1, 2, 3, 4;
 ;;
 print $_;
"
991 992 993 994
881
882
883
884
same as it ever was
[download]

(For some insight into the difference between a list and an array, try executing the statement
    $_ = $_ + 990 for 1, 2, 3, 4;
instead.)