Am i correct in summarizing the logic as you want to include the image of latest date that is on or before the given date? I think you have the general idea, though no need to read in an array .. just loop backwards with a date module (Date::Calc or any of the others) and check if that file exists. Something like (warning -- untested):

use Date::Calc qw/ Today Delta_Days Add_Delta_Days /;
my ($start_date, $end_date) = ('2005-04-25', '2005-05-05');
my @target_date = Today();
my @start_date = split /-/, $start_date;
my @end_date = split /-/, $end_date;
die "not in range" unless Delta_Days(@start_date, @target_date)>=0 && 
+Delta_Days(@target_date, @end_date)>=0;
die "bad order" unless Delta_Days(@start_date,@end_date)>=0;
my @d = @target_date;
my $filename = '';
while( Delta_Days(@start_date,@d)>=0 ){
  my $f = sprintf "%04d-%02d-%02d.jpg", @d;
  if( -f $f ){
    $filename = $f;
    last;
  }
  @d = Add_Delta_Days(@d, -1);
}
print "File: $filename";
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