Sure. This is the same subroutine but opened out and using named parameters to explain the intermediate steps.

sub rndStr{
    # When an array is interpolated in a string $" is used to seperate
+ the values 
    # By setting it (locally) to '', we ensure that the string doesn't
+ have spaces or 
    # anything else inserted between the characters.
    local $" = ''; 
    
    my ( $len, @chars_to_pick_from) = @_;
    
    # @picks will end up with as many random values as requested.
    my @picks = map{ 
                    # Generate random numbers bewteen 0 and 
                    # the number of chars supplied to pick from
                    rand $#chars_to_pick_from
                    
                    # do it as many times as request
                   } 1 .. $length;
                   
    # Using an array slice interpolated in quotes to construct the str
+ing
    # @picks contains the random indices, the slice picks them out of 
+the
    # array of choices past in. The resultant string is returned as a 
+scalar.
    "@chars_to_pick_from[@picks]"; 
}
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In the short version above, I'm just using the @_ array, (and shift to get the first parameter) and doing the whole thing in a single step rather than all seperated out.

Hope that helps.