You can sort the dates lexically if you reverse the day, month and year and then just grab the first one, like this

use strict;
use warnings;

my @startDates =
   qw(01-06-2007 01-08-2006 01-06-2006 01-07-2007 06-01-2007);

my $leastDate =
   (
      map  {join q{-}, reverse split /-/}
      sort
      map  {join q{-}, reverse split /-/}
      @startDates
   )[0];

print qq{Least date is $leastDate\n};
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When run this produces

Least date is 01-06-2006
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I hope this is of use.

Cheers,

JohnGG