$ cat foo.pl
use strict;
use warnings;
my $x; my $y;
while (<DATA>) {
    chomp;
    (2==0+(($x, $y) = split /:/,$_)) or print "wrong number of fields\n";
    print "<$_>, <$x>, <$y>\n";
}
__DATA__
FOO:BAR:BAZ:PLOVE
FOO:BAR:BAZ
FOO:BAR
BAZ
$ perl foo.pl
wrong number of fields
<FOO:BAR:BAZ:PLOVE>, <FOO>, <BAR>
wrong number of fields
<FOO:BAR:BAZ>, <FOO>, <BAR>
<FOO:BAR>, <FOO>, <BAR>
wrong number of fields
Use of uninitialized value $y in concatenation (.) or string at foo.pl line 7, <DATA> line 4.
<BAZ>, <BAZ>, <>