Stop me if I'm saying something obvious, but does the plan of your script commit you to this data structure? - if not, would there be any merit in considering an array of hashes? - each hash having the three keys user, date, file. Then it would be easy to grep out the bits you want, e.g.

@results = grep {
    $_->{user} eq 'Fred' and
    $_->{date} >= 1023318000 and
    $_->{date} <= 1024182000
} @$userdatefile;
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to get everything by Fred between the 6th and 16th of this month.

§ George Sherston