open IN, "<", "file.txt" or die "Cannot open file.txt for reading:$!\n";
my @lines = <IN>;
$lines[0] =~ s/,+$/,/;
$lines[-1] =~ s/,+$/,/;
open OUT, ">", "file.txt" or die "Cannot open file.txt for writing:$!\n";
print OUT for @lines;
close OUT;