s/^(.)(.)(.*)/$2$1$3/s;           # 12345 -> 21345
s/^(.)(.)(.)(.*)/$1$3$2$4/s;      # 12345 -> 13245
s/^(..)(.)(.)(.*)/$1$3$2$4/s;     # 12345 -> 12435
s/^(...)(.)(.)(.*)/$1$3$2$4/s;    # 12345 -> 12354