I suggest you spend a bit more effort on naming things better (and that you use strict when it doesn't hinder you, but that's another matter). As an example, try reading the main loop in this:

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

# untested; merely to prove a point

use GD;
use File::Spec;

use constant FONT_PATH => '/full/path/to/fonts/dir/';

use constant HEIGHT => 800;
use constant WIDTH => 320

use constant COL_WIDTH => 195;
use constant LINE_HEIGHT => 20;

use constant LEFT => 5;
use constant TOP => 20;

my $im = GD::Image->(WIDTH, HEIGHT);
$im->colorAllocate(255,255,255);

my $black = $im->colorResolve(0,0,0);

my @font_name = do {
    opendir my($dh), FONT_PATH
        or die "Couldn't open ", FONT_PATH, ": $!\n";
    grep { $_ ne "." and $_ ne ".." } readdir $dh;
};

my $x = LEFT;
my $y = TOP;

for (@font_name) {
    if($y > HEIGHT) {
        $x += COL_WIDTH;
        $y = TOP;
    }

    my $ttf = File::Spec->catfile($fontDir, $_);

    $im->stringTTF($black, $tff, 12, 0, $x, $y, $_)
        or die "Failed trying to draw $_: $!\n";

    $y += LINE_HEIGHT;
}

open my $fh, ">", "fonts.png"
    or die "Couldn't open fonts.png: $!\n";

binmode $fh;
print $fh $im->png;
[download]

In your code, it's a lot less obvious what's really going on. I like to believe that the renamed variables and the constants here made it extremly clear what's going on. Also, it's easy to find what "special values" a program uses that way, and they're also easy to customize.

Also, try to use lexical filehandles wherever possible. It just makes sure you'll never accidentally step on anyone else's foot.

Of course this is just a small script and wouldn't really justify the critique, but these things become second nature and happen without thinking even when you're writing small scripts, if you've gotten into the habit. And the habit will serve you well in large projects.

Makeshifts last the longest.