From the
perlform manpage:
Picture fields that begin with ^ rather than @ are treated
specially. With a # field, the field is blanked out if
the value is undefined. For other field types, the caret
enables a kind of fill mode.
So change your
@#### to
^####, and make sure the values you want to suppress are undefined.
The second problem is that using an undefined value when warnings are enabled will generate several warning messages. I suggest disabling warnings within the format, like this:
{ no warnings;
format STDOUT =
@<<<<<<<<<<<<< ^#### @#####.##
$output[$i], $output[$i+1], $output[$i+2]/1024
.
}