#!/usr/bin/env perl -l

use strict;
use warnings;

my @strings = (
    '8 changes by 4 users: user1 user2 user3 user4',
    '1 change by user1',
); 

for (@strings) {
    my ($changes, $users, @user_list);

    if (/:/) {
        /^(\d+)\D+(\d+)[^:]+:\s+(.+)/;
        $changes = $1;
        $users = $2;
        @user_list = split /\s+/ => $3;
    }
    else {
        /^(\d+) change by (\w+)/;
        $changes = $1;
        $users = 1;
        @user_list = ($2);
    }

    print "changes=$changes; users=$users; user_list: @user_list";
}