Here’s another way:

If you don’t need the original string anymore, just give an empty string as the REPLACEMENT argument to substr:

 1:26 >perl -E " $string = qq[ATATTTATATTAT]; say $string; $removed = 
+substr($string, 0, 3, q[]); say $removed; say $string; "
ATATTTATATTAT
ATA
TTTATATTAT

 1:30 >
[download]

Hope that helps,