Firstly, do you have a timestamp in the name of those files? If so, you can use glob and regex the filenames...

If not, you can use this:

use strict;
use warnings;
use File::Glob;

my $path = "";
my ($oldest, $fname) = (time, '');
my $modtime;

chdir $path;
foreach (glob("*")) 
{
   if (-f)
   {
      $modtime = (stat($_))[10];
      ($oldest = $modtime and $fname = $_) if ($oldest > $modtime);
   }
}
print "$fname\t$oldest\n";

(abs(time() - $oldest) < 300) ? print "File recent enough\n"
                              : print "Reporting an error\n";
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This will go through files in a directory ($path), find the oldest file, check if it was modified (created or overwritten) within last 300 seconds... the rest insert accordingly.