use Modern::Perl;
use List::Util qw( max );

# Build a sample dataset of ten rows of ten columns each,
# with random numbers between zero and nine.

my @lists = map { 
    [ map { int( rand( 10 ) ) } 1 .. 10 ]
} 1 .. 10;


# Invoke our dirty little function to get the list
# of maxes per row, and columns that hit that max.

my @max_indices = get_max_and_column_list_per_row( @lists );


# Print our list out nicely.

{
    local $" = ", ";
    foreach my $row_index ( 0 .. $#max_indices ) {
        my ( $max, $columns ) = each %{ $max_indices [ $row_index ] };
        say "Row  $row_index  has a max of  $max  in column(s)  @{$columns}";
    }
}


# The ugly function.  Accepts a LoL, returns a LoHoL.

sub get_max_and_column_list_per_row {
    my( @arrays ) = @_;

    # First, find out how many columns we have.
    my $top_index = max( map $#{$_}, @arrays );

    # The rest is a big map{}.
    return map {
        my $outer_index = $_;
        my $current_max = max( @{ $lists[$outer_index] } );
        +{  # Anonymous hash constructor.
            $current_max => 
            [ grep { $lists[$outer_index][$_] == $current_max } 
              0 .. $top_index ] 
        } # End of anonymous hash constructor.
    } 0 .. $#lists;
}

##</code><code>##

Row  0  has a max of  9  in column(s)  2, 5
Row  1  has a max of  7  in column(s)  7, 9
Row  2  has a max of  7  in column(s)  6
Row  3  has a max of  9  in column(s)  1
Row  4  has a max of  9  in column(s)  3
Row  5  has a max of  9  in column(s)  8
Row  6  has a max of  6  in column(s)  5, 6, 7, 8
Row  7  has a max of  9  in column(s)  7
Row  8  has a max of  8  in column(s)  0
Row  9  has a max of  9  in column(s)  0