Note that glob '*' won't honour dotfiles. And why sort in the first place when you just want the oldest file? Using the convenient List::Util module you can emulate the structure of a Schwartzian Transform without sorting in this case:

use List::Util qw(reduce);
opendir DIR, ".";
print @{
    reduce { $a->[1] < $b->[1] ? $a : $b }
    map { -f $_ ? [$_, (stat _)[9]] : () }
    readdir DIR
}->[0], $/;
closedir DIR;
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And without a module:

opendir DIR, ".";
my ($oldest, @file) = map { -f $_ ? [$_, (stat _)[9]] : () } readdir D
+IR;
closedir DIR;

while(my $cur = shift @file) {
    $oldest = $cur if $oldest->[1] > $cur->[1];
}

print $oldest->[0], $/;
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Makeshifts last the longest.