Using unpack and a hash instead of regex could be cool too:

use strict;

my %pat = ( abc => 1,
            xyz => 1 );

while (<DATA>) {
    my $out = '';
    for my $seg ( unpack( 'A3A3A3', $_ )) {
        $out .= "$seg " if ( $pat{$seg} );
    }
    print "$.: $out\n" if ( $out );
}

__DATA__
abcmmmnnn
mmmxyznnn
mabcmxyzm
xyzabcmmm
abcmxyznn
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