$ for i in `seq 1 13`; do echo Line $i; done | perl -e '
while ( not eof STDIN )
{
    my $buf;
    $buf .= $_ for map { eof STDIN ? () : scalar <> } 1 .. 5;
    print $buf;
    print q{+} x 10, qq{\n};
}'
Line 1
Line 2
Line 3
Line 4
Line 5
++++++++++
Line 6
Line 7
Line 8
Line 9
Line 10
++++++++++
Line 11
Line 12
Line 13
++++++++++
$