use Fcntl qw(LOCK_EX LOCK_UN);

my $header = 'foo';
my $footer = 'bar';
my $prefix = 'baz';

open CSV,$csvfile;
open OUTPUT,">$header";
flock OUTPUT,LOCK_EX;

$, = ","; # output field seperator, joins lists passed to print

while (<CSV>){
    print OUTPUT $prefix,$_; # no need to split;
            # just so you know : @list = split(/,/,$_);
            # will do the trick better than $var1,$var2...
            # but you want to chomp($_) first, and set $\ to "\n" perhaps
            # then you print OUTPUT $prefix,@list
            # and CSV is done for you.
}

print OUTPUT $footer;

flock OUPTUT,LOCK_UN; # not really needed, i think
close OUTPUT;
close CSV;