use warnings;
use strict;

my $infile="clients.txt";
open my $in,  "<", $infile     or die $!;
open FH, ">" , "clients.csv" or die $!;


print FH join ("," => qw(Name Phone E-mail)),$/;
while (<$in>) {
    chomp;
    print FH $_, "\t";
    print FH $/ if $. % 3 == 0; # note here 
}