; String variable can be defined by appending .s to its name during definition or by appending and using $ as a part of its name.
Define txt$, txtvar.s="little"

;Load datasegment into variable txt$
Restore Mary
Read.s  txt$

; Replace X with "little" and store result in txt$
txt$=ReplaceString(txt$,"X",txtvar)

OpenConsole(): Print(txt$)

DataSection:
  Mary:
  Data.s  "Mary had a X lamb."
EndDataSection