How to truncate a string to a specific length in perl?

You want to use the substr() function.

$shortened = substr( $long, 0, 50 ); # 50 characters long, starting at the beginning.

For more, use perldoc

perldoc -f substr

In your case, it would be:

$str="abcd";
$short = substr( $str, 0, 3 );