Not to complicate matters, but you could also use stat(). Using stat() is particularly useful if you want to see the actual datestamp that the file was last modified, whereas -M only returns the number of days since modification. It is possible to get get the actual datestamp if you've only used -M, but you'd have to use Date::Manip. To do the whole process using stat(), you could do it something like this:

$sevenDays = 60 * 60 * 24 * 7;
$now = time();
foreach $file ( @files ) {
    $mod_time = ( stat($file) )[9];
    if ( $now - $sevenDays < $mod_time ) {
        # Our file has been modified w/i last 7 days
    } else {
        # Hasn't been modified w/i last 7 days
    }
}
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It is then a trivial matter to retrieve the exact date of the modification, like so:
$date = localtime( (stat($file) )[9] );
Or, using the first example, since we've already got the modification time stored in $mod_time, we'd simply have to use $date = localtime( $mod_time ).