use strict;
use warnings;

my %hash;

while (defined (my $line = <DATA>)) {
    chomp $line;
    
    my ($key, $value) = split '', $line, 2;
    
    next if ! defined $value;
    
    $hash{$key} = $value if ! exists $hash{$key} || $hash{$key} < $value;
}

print "$_: $hash{$_}\n" for sort keys %hash;

__DATA__
a 1
b 3
a 6
c 4
a 2

##</code><code>##

a:  6
b:  3
c:  4