while ( <DATA> ) {
  if ( /^(\d+\/123 11.*?)(?= a )(?:.*?zyx(.*))?/ ) {
    print "THIS IS THE FIRST MATCH: $1" ,$/;
    print "THIS IS THE SECOND MATCH: $2" ,$/ if defined $2;
  }

}

__DATA__
12/123 11-abc 456 a 1/2 zyx this is the last 1
13/123 11-abc 456 a 1/2 zyx this is the last 2
14/456 11-abc 456 a 1/2 zyx this is it
15/456 11-abc 456 a 1/2 zyx this is it
16/123 11-abc 456 a 1/2 zyx this is the last 3
17/123 11-abc 456 a 1/2 xyz this is the last 3