Perl Cookbook item 7.5: Creating Temporary Files.

use IO::File;
use POSIX qw(tmpnam);

my $name;

do{ $name = tmpnam() 
} until $fh = IO::File->new( $name, O_RDWR|O_CREAT|O_EXCL );

binmode $fh;
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Use $fh as the file handle, instead of IMG

Don't forget to change the file name in the <IMG SRC= .. print statement. You'll need some way to delete old files every once in a while.