I'm not getting any errors just using the snippets you posted above. Can you give us the exact message it gives you when you fail? And, maybe post the second version of $cmd in context showing us how it's used? Specifically, I don't have nmake, so I wrote a little program to show the arguments it receives instead. I get this output using your snippet above:

use strict;
use warnings;

my $cmd=qq|showargs NODEBUG=1 "ADDED_CFLAGS=/DCOST_EN_US /DPAYMENT_CRE
+DIT_CARD"|;
print "$cmd\n";

open(RS,"$cmd 2>&1 |") or die "Can't open pipe: $!\n";
while (<RS>) {
        print;
}
close(RS);
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Here's the output, showing the complete quoted second argument you had:

showargs NODEBUG=1 "ADDED_CFLAGS=/DCOST_EN_US /DPAYMENT_CREDIT_CARD"
Arguments were:
    0: NODEBUG=1
    1: ADDED_CFLAGS=/DCOST_EN_US /DPAYMENT_CREDIT_CARD
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