my %file = ( name => 'name', date => '0000-00-00' );
my $dir  = 'somedir';

opendir DIR, $dir or die "Cannot open '$dir' $!";

while ( my $file = readdir DIR ) {
    next unless $file =~ /(\d{4}-\d\d-\d\d)\.txt$/;
    if ( $1 gt $file{ date } ) {
        @file{ 'name', 'date' } = ( $file, $1 );
        }
    }

closedir DIR;

print "Oldest file is $file{name}.\n" if $file{ date } ne '0000-00-00';