sub alist_get {
  my $k = shift;
  for (@_) { return $_ if $_->[0] eq $k }
  die "No such key in alist: $k"
}

push @{alist_get('two', @arr)->[1]}, 222;
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Incidentally, => works just like a comma, except that it stringifies its LHS. It's not hash-related magic. So, you could define your array thus:

And likewise I could've invoked my subroutine as alist_get(two => @arr) -- Neat, huh?

/assumes you have a good reason to not use a hash. :-)