%r ends up having the keys (a,b,c,d,e,f,g,h,i,j) and their respective values (A,B,C,D,E,F,G,H,I,J)

The line @A{keys %r} = keys %r; causes %A to have the keys (a,b,c,d,e,f,g,h,i,j) and the respective values (a,b,c,d,e,f,g,h,i,j).

In the final line, print %A, "\n";, %A is being used in a list context, and thus looks like a list of key-value pairs, like so: (a,a,b,b,c,c,d,d,e,e,f,f,g,g,h,h,i,i,j,j), which gets printed out with no delimiters between each element as aabbccddeeffgghhiijj, which is what you saw.

That help?