I quoted the entire line 51 and the problem went away. Just so that's clear, checking syntax of:

...
format STDOUT_TOP = 
@<<<<<<<<<<<<<<<     @||||||||||    @|||||    @|||||
q{User                  Command        CPU%      Mem%}
.
$^ = 'STDOUT_TOP';
...
[download]

gives

$ perl -c pm_format_syntax.pl
pm_format_syntax.pl syntax OK
[download]

I haven't used format in almost 15 years, so I can't give a definitive reason as to why this fixed the problem. My rationale for trying the quoting was the presence of the % characters in the line immediately prior to the line which generated the error. Other monks may provide a more detailed explanation.