Assuming you have a Hash-of-Arrays:

use strict;
use warnings;
use List::Util qw(max);

my %hoa = (
    a => [(1..3)],
    b => [(4..6)]
);

for (sort keys %hoa) {
    print "$_ ", max(@{$hoa{$_}}), "\n";
}

__END__
a 3
b 6
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