When dealing with fixed length records, it was recommended to use pack / unpack pair. See "Perl Best Practices" by Damian Conway for more. With this, here is another solution that reflects this recommendation:

use strict;
use warnings;

my $spat = qr/(?:abc|xyz)/;

while(<DATA>) {
        chomp;
        my $r = $_;
        my @flds = unpack "(A3)*", $r;

        print "$r => ";
        do { print "$_ " if /$spat/ } for @flds;
        print "\n";
}
__DATA__
abcmmmnnn
mmmxyznnn
xyzfooabc
mabcmxyzm
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-BR