use Fcntl 'SEEK_SET';
open my $fh, "+<", $file or die;
local $/;
local $_ = <$fh>;
s/,(?=\n|$)//g;
seek $fh, 0, SEEK_SET or die;
print $fh $_ or die;
truncate $fh, tell $fh or die;
close $fh or die;