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