sub tree_data {
  my ( $old_data, $key_name, $next_group, @remaining_keys ) = @_;
  my ( @new_data, $last_key, $build_box );

  foreach ( @{ $old_data } ) {
    if ( $last_key and ( $last_key ne $_->{ $key_name } ) ) {
      push @new_data, {
        $key_name   => $last_key,
        $next_group => ( @remaining_keys )
          ? tree_data( $build_box, @remaining_keys )
          : $build_box,
      };
      undef $build_box;
    }
    $last_key = delete $_->{ $key_name };
    push @{ $build_box }, $_;
  }
  push @new_data, {
    $key_name   => $last_key,
    $next_group => ( @remaining_keys )
      ? tree_data( $build_box, @remaining_keys )
      : $build_box,
  };

  return \@new_data;
}

##</code><code>##

my $input = [
  {
    'team'       => 'A-Team',

    'employee'   => 'Face',
    'work_day'   => '2006-08-28',
    'other_data' => '123456789',
  },
  {
    'team'       => 'A-Team',
    'employee'   => 'Murdock',
    'work_day'   => '2006-08-28',
    'other_data' => '123456789',
  },
  {
    'team'       => 'Military',
    'employee'   => 'Decker',
    'work_day'   => '2006-08-28',
    'other_data' => '123456789',
  },
];

##</code><code>##

my $output = [
  {
    'team'      => 'A-Team',
    'employees' => [
      {
          'employee'   => 'Face',
          'work_day'   => '2006-08-28',
          'other_data' => '123456789',
      },
      {
          'employee'   => 'Murdock',
          'work_day'   => '2006-08-28',
          'other_data' => '123456789',
      },
    ],
  },
  {
    'team'      => 'Military',
    'employees' => [
      {
          'employee'   => 'Decker',
          'work_day'   => '2006-08-28',
          'other_data' => '123456789',
      },
    ],
  },
];

##</code><code>##

# ...to generate the output example from above...
my $output = tree_data( $input, 'team', 'employees' );

# ...to go a little deeper...
my $output = tree_data(
  $input, 'team', 'employees', 'employee', 'work_days'
);