I'm almost certain that interpolation of arrays was available in perl4. Hmm... it was. I just tested it with perl 4.036.

See the first paragraph in this document describing traps when going from perl4 to perl5.

@ now always interpolates an array in double-quotish strings.

Note the word "always". It used to be so that perl4 only interpolated arrays where they existed before being used in a string.