use POSIX qw(ceil);

my $rows = ceil( scalar @a / $col );
for ( my $i = 0; $i < $rows; $i++ ) {
   printf( ("%-${max}s " x $col)."\n", map { defined $_ ? $_ : '' } 
      @a[($i*$col)..(($i*$col)+($col-1))] );
}

# transposed
$rows = ceil (scalar @a / $col );
for ( my $i = 0; $i < $rows; $i++ ) {
  printf( ("%-${max}s " x $col)."\n", map { defined $_ ? $_ : '' }
    @a[ map { $i+($_*$rows) } (0..$col-1) ] );
}