A nested hash structure is probably the best solution:

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

my %sum;

while (<DATA>) {
    chomp;
    
    my ($key, $value, $reason) = split(/\|/);

    if (! defined $reason || $value !~ /^\d+$/) {
        warn qq<dropped line: "$_"\n>;
        next;
    }

    $sum{$key}{value} += $value;
    push @{$sum{$key}{reason}}, $reason;
}

local $" = '/';
for my $key (keys %sum) {
    print "$key|$sum{$key}{value}|@{$sum{$key}{reason}}\n";
}

__DATA__
USERID1|2215|Jones,Tom|
USERID1|1000|Jones, Tom|
broken line
USERID3|1495|Dole, Bob|
USERID2|2500|Francis, Pope|
USERID2|1500|Francis, Pope|
[download]

Prints:

dropped line: "broken line"
USERID3|1495|Dole, Bob
USERID1|3215|Jones,Tom/Jones, Tom
USERID2|4000|Francis, Pope/Francis, Pope
[download]

There are a few coding things to watch out for:

• Don't declare a bunch of variables in a list. That's often a code smell that says "just doing this to shut up strictures, but I really don't know why I need to". Instead declare variables where they are first needed. Ideally declare a variable where it is initialized.
• Don't initialize arrays and hashes with an empty list. They are empty out of the box.
• Avoid unless and until. If their expressions are anything other than completely trivial they become almost impossible to understand.
• Don't put multiple statements on a single line and don't put the block for a loop or if statement on the same line as the loop/if.