my @max;
foreach ( keys %ENV){
    push @max, length $_;
}
my @sorted_max = sort { $a <=> $b } @max;
my $max_num = pop @sorted_max;

while ( my ($key, $value )= each %ENV) {
    $value = $value // 'undefined value'; 
    printf "\n%-*s: %s", $max_num, $key, $value;
}