The syntax is OK:

X:\>perl -c tmp.pl
tmp.pl syntax OK

X:\>type tmp.pl
#! usr/bin/perl -w
use strict;


use File::Copy;
use File::Path;

my $date = $ARGV[0];

my $comptage = $ARGV[1];
my $C = $comptage;

# index : input ou output
my $index = $ARGV[2];
my $I = $index;

my $week = $ARGV[3];
my $W = $week;

print STDOUT "the week is ${W}\n";


my $cmda=qq{mkpath('M1_fa_${W}/M1_${W}_comptage_${C}_${I}_first_action
+')};
my $cmdb=qq{mkpath('M2_fa_${W}/M2_${W}_comptage_${C}_${I}_first_action
+')};
my $cmdc=qq{mkpath('M3_fa_${W}/M3_${W}_comptage_${C}_${I}_first_action
+')};
[download]

Maybe you want to be more explicit about where your problem is, that is, what your program should do, what it does, and how that differs from what you expect.

Maybe it is already enough for you to read the quotes section in perlop, Quote and Quote-like Operators.