system ('mail -s "Test" testmail@server.com, < DailyReport_$d.txt');
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Do you get a subject if you issue the same command at the shell prompt?

But this might probably be more a Unix/Linux question than a Perl question. Not all mail utilities have exactly the same syntax, may be your should check yours.

A possible alternative might be something along these lines (I haven't tested exactly that, but only something similar):

my $subject = "Mail subject\n";
my $from = 'me@my_company.com\n';
my $cc = "whoever\@my_company.com\n";                    

open my $MAIL, "|/usr/sbin/sendmail -t" or die "... $!";;
    
# Mail Header
print $MAIL "To: addressee\@my_company.com\n";
print $MAIL "Cc: $cc";
print $MAIL "From: $from";
print $MAIL "Subject: $subject\n\n";

# Mail Body
print $MAIL "\nThe accompanying message to be sent.\n\n";
my $stat_file = "$result_path/STAT.TXT";
open my $COUNT, "<", $stat_file or die "can't open $stat_file $!";
print $MAIL <$COUNT>;
close $COUNT;
print $MAIL "\n\n\n";
print $MAIL "Regards, \n";
print $MAIL "sender's signature, \n";
close $MAIL;
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Having said that, it might well be better to use a mail module, such as Mail::Send, Mail::Sendmail or some other (there are many, and I really can't advise one against any other).

I don't know why the subject wouldn't be set, but there are problems with your snippet as shown:

Single quotes will not interpolate. I suggest you split up the line as:

my $cmd = 'mail -s "Test" testmail@server.com, < DailyReport_$d.txt';
print "Running [$cmd]\n";
system($cmd) == 0
    or warn "Couldn't run [$cmd]: $! / $?";
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That way, you will see that $d does not interpolate in your command line.

You should swap the quoting style and always quote all the components:

my $cmd = "mail -s 'Test' 'testmail\@server.com' < 'DailyReport_$d.txt
+'";
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