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.
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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
.
}
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