Store the string MSA on the string stack in the variable
whose DFA is on the parameter stack.
NOTES: The only situation in which $! copies the string
value is when it is a bound string already stored in another
variable. In that case, the new copy is the one that is
stored in the variable. In particular, external strings are
If the string value held by the string variable on entry is a
bound string that is also referenced deeper on the string
stack, its back link is reset to point to the deepest string
stack reference. If it is a bound string not deeper on the
string stack and not identical to the input string, its back
link is set to zero, making it garbage. If it is an external
string, its MSA in the variable is simply written over by
that popped from the string stack.