Nice work. To shorten it without obfuscating it, you can remove a lot of repeated calculations. Compare:

sub minkowski {
    my ($d, $level) = @_ ;
    if ($level==0) {fd($d); return 1;} 
    minkowski($d/4,$level-1);
    rt(-90); minkowski($d/4,$level-1);
    rt( 90); minkowski($d/4,$level-1);
    rt( 90); minkowski($d/4,$level-1);
    minkowski($d/4,$level-1);
    rt(-90); minkowski($d/4,$level-1);
    rt(-90); minkowski($d/4,$level-1);
    rt( 90); minkowski($d/4,$level-1);
}
[download]

Now each calculation done once:

sub minkowski {
    my ($d, $level) = @_ ;

    fd($d), return unless $level--;

    $d /= 4;

    minkowski($d,$level);
    rt(-90); minkowski($d,$level);
    rt( 90); minkowski($d,$level);
    rt( 90); minkowski($d,$level);
    minkowski($d,$level);
    rt(-90); minkowski($d,$level);
    rt(-90); minkowski($d,$level);
    rt( 90); minkowski($d,$level);
}
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That didn't buy much here because the function name is so long. Then, notice how you often have rt($foo); minkowski(@bar); ? You could shorten that by making the angle an optional parameter to the second function.

sub minkowski {
    my ($d, $level, $rt) = @_ ;

    rt($rt) if $rt;
    fd($d), return unless $level--;

    $d /= 4;

    minkowski($d,$level);
    minkowski($d,$level, -90);
    minkowski($d,$level,  90);
    minkowski($d,$level,  90);
    minkowski($d,$level);
    minkowski($d,$level, -90);
    minkowski($d,$level, -90);
    minkowski($d,$level,  90);
}
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Hmm. We're calling the same function eight times in a row, with only one different parameter. Looks like an unrolled loop, no?

sub minkowski {
    my ($d, $level, $rt) = @_;

    rt($rt) if $rt;
    fd($d), return unless $level--;

    $d /= 4;
    minkowski($d, $level, $_)
        for 0, -90, 90, 90, 0, -90, -90, 90;
}
[download]

Makeshifts last the longest.