I don't know whether you want to catch the string before zyx, using the /g and \G may not be necessary if you don't care about it, although it would ensure that you get the right zyx. How about something like:

$line[0]="12/123 11-abc 456 a 1/2 zyx this is the last";
$line[1]="14/456 11-abc 456 a 1/2 zyx this is it";

for (@line)
{
        if(m|(\d+/123 11-[a-z]+ \d+)|g)
        {
                $start=$1;
                /\G(.*?) zyx (.*)$/;
                print "Start of record:$start\nBefore zyx:$1\nand Afte
+r zyx:$2\n";
        }
}

__OUTPUT__

Start of record:12/123 11-abc 456
Before zyx: a 1/2
and After zyx:this is the last
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