Splitting into letters seems more cumbersome than using regular expressions:

use strict; use warnings;
my @lexikon = qw/ own known /;
my $word = "unknown";

my $longestmatch = "";
foreach my $w (@lexikon) {
    if( $word =~ /$w$/ ) {
    $longestmatch=$w if length($w)>length($longestmatch);
    }
}
# $longestmatch now has the longest match
$word =~ /(.*)($longestmatch$)/;
print "$1+$2\n";
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