A simple (but inefficient) way is:

$_ = ' RAGGTS ';

while (/[RS]/)
{   s/ ([A-Z]*)R([A-Z]*) / $1A$2 $1G$2 /g;
    s/ ([A-Z]*)S([A-Z]*) / $1C$2 $1G$2 /g;
}

print;
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