#!/usr/bin/perl

use strict;
use warnings;

foreach (<DATA>) {
  chomp;
  print "$_ : ";
  if (/^(?:...)*(abc|xyz)/) {
    print "Match on $1\n";
  } else {
    print "No match\n";
  }
}

__DATA__
abcmmmmmm
mmmxyzmmm
mabcmxyzm
[download]

while(<DATA>) {
    chomp;
    print "$_ => @_\n" if @_= /(abc|xyz)(?=(?:...)*$)/g;
}
__DATA__
abcmmmnnn
mmmxyznnn
mabcmxyzm
xyzfooabc
__END__
abcmmmnnn => abc
mmmxyznnn => xyz
xyzfooabc => xyz abc
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

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
[download]

-BR

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
[download]