I am finding hard to understand your response. The logic I am trying to apply is

You are trying to hard to use pushes into arrays when what you really want to do is set up what I've called in the past a "keyed abacus." Use your date/time information as a key to a hash of hashes such that each element in the hash is a hash consisting of {min, max} values. Iterate through your data, create a key into your has with the date and time concatenated, if that key already exists update the min and max elements of the hash with the new values.

Other than writing the code out for you long hand I'm not sure how much better I can explain it.

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

my $table={};
while(my $line=<DATA>){
     chop $line;
     my ($name,$date,$time,$value)=split(/[\s]+/,$line);
     my $key = $date . "-" . $time; 
     if ( ! defined($table->{$key}) { 
          $table->{$key}={ name => '', min => 99999999, max => 0 };
     } 
     $table->{$key}->{min} = $value if $value < $table->{$key}->{min}
     $table->{$key}->{max} = $value if $value > $table -> {$key}->{max
+}
}

foreach my $key (sort keys %$table){
    my ($date,$time) = split("-",$key);
    printf "Date: %s   Time: %s   max = %d  min = %d\n",$date,$time,
               $table->{$key}->{max},
               $table->{$key}->{min};
}

exit(0);

__END__
fred            9/1/2011     15:00:00   50
mary            9/1/2011     15:00:00   0
john            9/1/2011     15:00:00   16
[download]

You should see an output something like:

Date: 9/1/2011 Max: 50  Min: 0
[download]

By the way...this looks like the sort of assignment I would give to my students for homework when I used to teach Perl.