I can't tell for sure since you didn't show your __DATA__ section, but since it's complaining about not having a string to split after chomp, it's probably running into a blank line in your __DATA__ section. You can test for that before splitting (see below). On your other question, yes, it's worth sorting the database names into an array, not because of how many databases there are, but because you don't want to sort them again for each of 100_000 records, as my pseudo-code did.

I went ahead and did a working version that pulls in the data and produces the per-hour report like I think you want it. I added a lot of comments, but feel free to ask about anything you don't understand. I think you should be able to add the per-minute section yourself (see the comments for where), based on how the per-hour section works.

Once you're comfortable with how it works, one way to make the printing nicer would be to calculate the width of each column (for printf) based on the maximum width of the items in that column. I didn't get into that here, to keep it simple.

#!/usr/bin/env perl
use 5.010; use strict; use warnings;

my %h; my %m; my %db; # per-hour hash, per-minute hash, database names

while(<DATA>){
    next unless /\w/;                                # skip blank line
+s
    my($datetime,$database,$speed) = (split)[1,2,3];
    my $ddhhmm = substr $datetime,0,19;              # substr works we
+ll here since the lengths are static
    my $ddhh   = substr $datetime,0,16;              # this one doesn'
+t include the minutes
    $h{$ddhh  }{$database} += $speed;                # add the speed t
+o this hour & database
    $m{$ddhhmm}{$database} += $speed;                # add the speed t
+o this minute & database
    $db{$database} = 1;                              # save the databa
+se name
}
my @db = sort keys %db; # sort and save database names as array since 
+we'll be looping through them many times

# HOUR SECTION START
# print out the per-hour stats
# starting with a header line
print "  collectionTime";
printf "%11s", $_ for (@db);   # print each database name as a header 
+taking 10 spaces
print "\n";                    # end of line

for my $key (sort keys %h){
    print $key;                             # print the date/hour key
    printf "%11s", $h{$key}{$_} for (@db);  # print the value for each
+ database that goes with this key
    print "\n";
}
# HOUR SECTION END

# MINUTE SECTION START (using %m instead of %h)
# MINUTE SECTION END

__DATA__
server01: 2015-06-01T12:40:03-04:00  DB101                  10 MB/sec
server01: 2015-06-01T12:40:03-04:00  DB202                   5 MB/sec
server01: 2015-06-01T12:40:03-04:00  ASM                     2 MB/sec
server01: 2015-06-01T12:40:03-04:00  MYDB101                 2 MB/sec
server01: 2015-06-01T12:40:03-04:00  MYDB202                 5 MB/sec
server01: 2015-06-01T12:40:03-04:00  _OTHER_DB_             30 MB/sec
server01: 2015-06-01T12:41:03-04:00  DB101                   3 MB/sec
server01: 2015-06-01T12:41:03-04:00  DB202                   4 MB/sec
server01: 2015-06-01T12:41:03-04:00  ASM                     2 MB/sec
server01: 2015-06-01T12:41:03-04:00  MYDB101                 9 MB/sec
server01: 2015-06-01T12:41:03-04:00  MYDB202                 7 MB/sec
server01: 2015-06-01T12:41:03-04:00  _OTHER_DB_             50 MB/sec
server02: 2015-06-01T12:40:03-04:00  DB101                  90 MB/sec
server02: 2015-06-01T12:40:03-04:00  DB202                   9 MB/sec
server02: 2015-06-01T12:40:03-04:00  ASM                     2 MB/sec
server02: 2015-06-01T12:40:03-04:00  MYDB101                 3 MB/sec
server02: 2015-06-01T12:40:03-04:00  MYDB202                 1 MB/sec
server02: 2015-06-01T12:40:03-04:00  _OTHER_DB_             90 MB/sec
server02: 2015-06-01T12:41:03-04:00  DB101                   1 MB/sec
server02: 2015-06-01T12:41:03-04:00  DB202                   4 MB/sec
server02: 2015-06-01T12:41:03-04:00  ASM                     2 MB/sec
server02: 2015-06-01T12:41:03-04:00  MYDB101                 7 MB/sec
server02: 2015-06-01T12:41:03-04:00  MYDB202                 7 MB/sec
server02: 2015-06-01T12:41:03-04:00  _OTHER_DB_             55 MB/sec
[download]