You also have a syntax error -- the 2 print statements are missing the terminal semi-colon:
print old "$LAST_NAME\t$FIRSTNAME\n"
print old "\t$ID_NUM\t$BOOK\n"
print old "$LAST_NAME\t$FIRSTNAME\n";
print old "\t$ID_NUM\t$BOOK\n";
The second one isn't technically mandatory, since it is followed by the brace, but it's good programming practice.
Edit: The reply above from VinsWorldcom did not originally have the necessary semi-colon ';', but he has now quietly added it to make it appear that his code was correct from the beginning. However, it's still better programming practice (as I mentioned) to add it to both lines.
substr+lc crypt(qw $i3 SI$),4,5