splice @a, 0, 1;
#          |  |
#          |  +-- length
#          +----- offset
#

##</code><code>##

for (0..$#a) {
  if ($a[$_] eq 'a') {
    splice @a, $_, 1;
    last; # exit the for loop because only want to delete
          # the first match
  }
}

##</code><code>##

@a = map { /a/ ? () : $_ } @a;