perl -wne '
  $h{$1}++ if /bn(\d{4}),/ }{$,="\t"; print %h, "\n"
' datafile

##</code><code>##

perl -MO=Deparse -wne '
  $h{$1}++ if /bn(\d{4}),/ }{$,="\t"; print %h, "\n"
'