In that case a simpler more direct approach might work for you

#!perl 
use strict;

my $inpath  = "/AAA/BBB/CCC/IN";
my $outpath = "/AAA/BBB/CCC/OUT";

# check inpath files less than 60 mins 
# and more than 15 mins old
my $min_age = 0.25/24; # 15 mins in days
my $max_age = 1.00/24; # 1 hour in days

my $count =0;
my $ignore=0;
print "Scanning in dir $inpath .. \n";

opendir DIR, $inpath or die $!;
for (readdir DIR){
  # skip if not a file
  next unless (-f "$inpath/$_"); 
  
  # filter on age
  my $age = -M "$inpath/$_";
  if ( ($age > $min_age) 
    && ($age < $max_age) 
    && ( !/_ACK_/ ) ){
  
    # build outfile name
    (my $outfile = $_) =~ s/(\.xml)$/_ACK$1/;
    
    # check outfile exists
    if (-e "$outpath/$outfile"){
      print "$_ => $outfile exists\n";
    } else {
      print "$_ => $outfile does not exist\n";
    }
    
    ++$count;
  } else {
    ++$ignore;
  }
}    
closedir DIR;
print "$count files checked - $ignore files ignored\n";
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poj