open(ARGS, ">args$$") or die "args$$: $!";
print ARGS <<ENDARGS;
new
size 256,256
... etc
ENDARGS
close(ARGS);

open(FLY, "$flyprog -f args$$ |") or die "$flyprog: $!";
binmode(STDOUT);
print "Content-type: image/png\n\n";
print <FLY>;
close(FLY);
unlink("args$$");