use warnings;
use strict;

open my $fh, '<', 'file.txt'
  or die "can't open the damned file!: $!";

while (<$fh>){
    chomp;

    if (! eof){
        print "$_,\n";
    }
    else {
        print "$_\n";
    }
}

##</code><code>##

one
two
three

##</code><code>##

one,
two,
three