Ah! Thanks!
I changed these lines ...

open( FH, '>image.png' ) or die "Can't image.png:$!\n";
.
.
.
<img src='image.png'>
[download]

... to ...

open( FH, '>../public_html/image.png' ) or die "Can't image.png:$!\n";
.
.
.
<img src='/image.png'>
[download]

... and now it works! Thanks.