The problem is that sort @a returns a list, not an array. What you've written is equivalent in syntax to shift (1,2,3). A read of perldata might provide some insight.

Update: In addition to ikegami's solutions below, you can use an anonymous array to make your code work. Pointless use of a shift, though, unless you store the array ref.

@array = ( 4, 1, 3, 2 );
print shift @{[sort @array]}; # Prints 1
print shift @{$sorted_ref = [sort @array]}; # Prints 1 and stores rema
+ining values
[download]