This one loops over the list of items once (at most). It computes the widths for all possible numbers of columns as it goes, eliminating cases of too-many columns when it determines that they would be too wide.

Fully tested and now it does not waste time on column counts over about @items/2 and (update 3) chooses single-row output correctly.

#!/usr/bin/perl -w
use strict;

my( $maxWid )= ( @ARGV, 79 );
my @items = qw(
    B ByteLoader Cwd DB_File Data Devel Digest
    DynaLoader Encode Errno Fcntl File Filter
    GDBM_File I18N IO IPC List MIME NDBM_File
    ODBM_File Opcode POSIX PerlIO SDBM_File
    Safe Socket Storable Sys Thread Time
    Unicode XS attrs re threads util
);
print '='x$maxWid, $/;
print Columns( $maxWid, \@items );
exit( 0 );

sub Columns
{
    my( $maxWid, $avItems )= @_;

    my $maxCols= 1 + $#$avItems/2;
    my @height= ( 0, map 1+int($#$avItems/$_), 1..$maxCols );
    my @total= ( $#$avItems, 0 .. ($maxCols-1) );
    my @width;
    for my $i (  0..$#$avItems  ) {
        my $len = length( $avItems->[$i] );
        $total[0] += $len;
        for my $cols (  1 .. $maxCols  ) {
            for(  $width[ $cols ][ $i/$height[$cols] ]  ) {
                $_ ||= 0;
                if(  $_ <= $len  ) {
                    $total[$cols] += $len - $_;
                    if(  $maxWid < $total[$cols]  ) {
                        $maxCols= $cols - 1;
                    }
                    $_= $len;
                }
            }
            last   if  $maxCols < $cols;
        }
        last   if  $maxCols < 2;
    }
    $maxCols ||= 1;
    my $height= $height[$maxCols];
    @width= @{ $width[$maxCols] };
    if( $total[0] <= $maxWid ) {
        $maxCols= @$avItems;
        $height= 1;
        @width= (0) x $maxCols;
    }
    my $text= '';
    for my $l (  1 .. $height  ) {
        my $i= $l - 1;
        my $c= 0;
        while(  $i < @$avItems  ) {
            my $item= $avItems->[$i];
            $i += $height;
            if(  $i < @$avItems  ) {
                $text .= sprintf "%-$width[$c++]s ", $item;
            } else {
                $text .= $item . $/;
            }
        }
    }
    return $text;
}
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