When you need to rename 5000 files from abcd_small.jpg to abcd.jpg, how do you do that?

  rename s/_small// *_small.jpg

When you have the string ABCD_XYZ in 50 files but you need to change it to DEFG_UVW, how do you do that?

  perl -pi -e 's{ABCD_XYZ}{DEFG_UVW}' **/*(.)

When you need to move all the files ending '.png' into the directory 'PNG', and all the ones ending '.jpg' into 'JPG', how do you do that?

  mkdir JPG PNG;
  for i in *.png *.jpg; mv $i $i:e:u