$/=undef;	# Slurps all data in file.
$_=<DATA>;	# Put it in $_, convenient for regex.
my $lines=0;	# Line count $. might suffice and then the pre increment can be dropped.
my $records=11;	# Number of lines in a record.
s/\n/(',',"\n")[++$line%$records==0]/eg;# replaces all \n with commas except the recordth.
print $_;