Here's one line that does exactly the same thing as your code using the ternary Conditional Operator.

my $in = ($scalar =~ /array/i) ? join(",",@$scalar) : $scalar;

Update: It occurs to me that by using a regex there, you are opening yourself up to a bug if $scalar contains the substring "array", so you should probably use the ref operator. And as Mutant points out below, if you are going for prettiness and legibility, an if-else is far clearer in intent than ? will be.