undefing @a would've cleared it, affecting one of the things that the symbol "b" also points to:

$ perl -le '@x = qw(1 2 3); $x = 1; *y = *x; undef @x; print "$y [@y]"
+'
1 []
[download]

but undefing *a clears the entries for "a" in the symbol table, without affecting the things that the symbol "b" also points to:

$ perl -le '@x = qw(1 2 3); $x = 1; *y = *x; undef *x; print "$y [@y]"
+'
1 [1 2 3]
[download]