thank you guys, I see the difference now

$grades{$student} .= $grade . " ";
./perl.score
$grades{lee} is 90 
$grades{lee} is 90 100 
$grades{kim} is 90 
$grades{kim} is 90 90 
kim: 90 90      Average: 90
lee: 90 100     Average: 95




$grades{$student} .= " " . $grade ;
[root@myserver chaos]# ./perl.score
$grades{lee} is  90
$grades{lee} is  90 100
$grades{kim} is  90
$grades{kim} is  90 90
kim:  90 90     Average: 90
lee:  90 100    Average: 95
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