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how to compare them

Compare what to what?

how to [...] print name of the biggest

use strict;
use warnings;
use feature qw( say );

use List::Util qw( reduce );

my %students = ( ... );

my $highest = reduce { $students{$a} >= $students{$b} ? $a : $b }
   keys(%students);

say $students{$highest};
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List::Util, say