This smells like homework.

You need a data structure to store, and lookup information.

I suggest using a hash:

my %student ; # content should be <id> => <name>
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After you read and split the first file, populate the hash:

$student{$def[2]} = {FIRSTNAME=>$def[0], LASTNAME=>$def[1]};
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When you read the grade, lookup and assign into the same structure:

$student{$def1[0]}{GRADE} = $def[1];
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(untested)