You'd want to check the modification time or date for all the files. Then you'd want to do a typical minimum function on the epoch times at which they were modified (using data from stat()) or a maximum function on the number of days since the file was last modified (using data from -M for example). Since the -X file test ops handle partial days, I usually just use those instead of stat if I only need one value.

A quick and dirty test for the oldest file in a directory follows. It currently makes no check to see if it's pointing to a regular file or something else (socket, directory, etc.). It just prints the name of the oldest file and the number of days since it was modified. I tested it a little and it seems to work.

use strict;
use warnings;

my $max = 0;
my ( $time, $oldest );
opendir ( my $d, '.' ) || die "Can't get file list: $!\n";
my @file = readdir ( $d );
closedir ( $d );
foreach ( @file )
{
    next if /^\.{1,2}$/;
    $time = -M $_;
    if ( $time > $max )
    {
        $max = $time;
        $oldest = $_;
    }
}
print "$oldest was modified $max days ago\n";
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