I liked your code, but I am averse to using [$index] in Perl code unless it is needed. I would give names to the list on the left as result of a split. A chomp() would make the split regex a bit easier, but no biggie. I am having a "brain cramp" right now on how to code the hash assignment statement better. My main point is to avoid this numeric $index stuff.

#!/usr/bin/perl -w
use strict;

my %records;
while (<DATA>) 
{
    my ($md5,$version,$end) = split(/[_\n]+/,$_);
    
    next unless ($end eq 'max');

    $records{$md5} = $version 
       unless (    exists $records{$md5} 
                && $records{$md5}< $version) ;                
}

foreach my $md5 (sort keys %records)
{
   print $md5,"_",$records{$md5},"_max\n";
}


#Prints:
#111aaa222ccc324567fed54333221235_1.02_max
#777aaa222ccc324567fed54333221235_1.04_max


__DATA__
111aaa222ccc324567fed54333221235_1.04
111aaa222ccc324567fed54333221235_1.05_max
111aaa222ccc324567fed54333221235_0.98_min
111aaa222ccc324567fed54333221235_1.02_max
111aaa222ccc324567fed54333221235_0.21
777aaa222ccc324567fed54333221235_1.04
777aaa222ccc324567fed54333221235_1.07_min
777aaa222ccc324567fed54333221235_1.04_max
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