Don't assign the return of print, just assign the value of the index array element:

my $field = $fields[0];
print "$field\n";
[download]

echo '2014-06-04T15:24:21-05:00 syslog_dp [0x80e00099][ftp][error] sec
+ure-backup(FBB): trans(5487)' | perl -lape '$_=$F[0]'
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)