This will loop a few times until it finds the best combination of lines/columns:

use strict;

my @a = sort qw(
B           Digest      Filter     MIME       SDBM_File  Time     util
ByteLoader  DynaLoader  GDBM_File  NDBM_File  Safe       Unicode
Cwd         Encode      I18N       ODBM_File  Socket     XS
DB_File     Errno       IO         Opcode     Storable   attrs
Data        Fcntl       IPC        POSIX      Sys        re
Devel       File        List       PerlIO     Thread     threads
);

sub try {
  my ( $n, @a ) = @_;
  my $lines = int ( 1 + ( @a + $n - 1 ) / $n );
  my @result;
  my $width = 0;
  for ( 1 .. $n ) {
    my @b = splice( @a, 0, $lines - 1 );
    my $colwidth = (sort( { $a <=> $b } map { 1+length $_ } @b ))[-1];
    push @result, [ $colwidth, @b ];
    $width += $colwidth;
  }
  return $width, @result;
}

my $SCREEN_WIDTH = 80;
my $col = 1;
my $last_width = 0;
my @result;
my $width;
my @last_result;
while(1) {
  ( $width, @result ) = try( $col, @a );
  last if $width > $SCREEN_WIDTH || $#{$result[0]} <= 1;
  $col++;
  $last_width = $width;
  @last_result = @result;
};

@result = @last_result if defined @last_result;
my @lines;
for my $col ( @result ) {
  my $width = shift @$col;
  $_ = sprintf( "%-${width}s", $_ ) for ( @$col );
}

for my $line ( 0 .. $#{$result[0]} ) {
  print $result[$_][$line] for 0 .. $#result;
  print "\n";
}
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Output (using $SCREEN_WIDTH=50 so that if fits here):

B          Encode    IPC       SDBM_File XS      
ByteLoader Errno     List      Safe      attrs   
Cwd        Fcntl     MIME      Socket    re      
DB_File    File      NDBM_File Storable  threads 
Data       Filter    ODBM_File Sys       util    
Devel      GDBM_File Opcode    Thread    
Digest     I18N      POSIX     Time      
DynaLoader IO        PerlIO    Unicode
[download]

update: changed one of the loop-end controls.