Somewhat daft but I notice that no one has mentioned the third argument to split yet:

use strict;
use warnings FATAL => 'all'; # ;)

my( $one, $two ) = split /:/, q[DATETIME 09:59:11], 2;

print substr( $one, -2 );
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From perldoc -f split:

If LIMIT is specified and positive, it represents the maximum number of fields the EXPR will be split into, though the actual number of fields returned depends on the number of times PATTERN matches within EXPR.