Note that your iterative solution can be slow with a long output list on any current Perl because iteratively building an array with unshift scales quadratically. (Fixed in the current development cycle.)

Given the pattern of unshift's and shift's in your code, you might or might not hit this problem. But still for a single reverse you can use the guaranteed efficient pop/push pair instead of shift/unshift:

sub flatten {
  my @flattened; # Will be in reverse order
  while (@_) {
    my $last = pop;
    if (UNIVERSAL::isa($last, "ARRAY")) {
      push @_, @$last;
    }
    else {
      push @flattened, $last;
    }
  }
  return reverse @flattened;
}
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