use strict;
use warnings;

sub GetINDirFiles {
        my ($path) = @_;
        opendir my $dir, $path or die $!;
        return grep {!/\_ACK_/} readdir $dir;
}

sub GetOUTDirFiles {
        my ($path) = @_;
        opendir my $dir, $path or die $!;
        return grep {/\_ACK.xml$/} readdir $dir;
}

# Main
my $inpath = "./IN";      
my $outpath = "./OUT";
my $outsuffix = "_ACK.xml";
my $insuffix = ".xml";     
my $timethreshold = 900;       # set time threshold in seconds (900 seconds equal 15 minutes)

my %outfiles = map { /(.*)\Q$outsuffix\E$/; $1 => 1 } &GetOUTDirFiles($outpath);
my @infiles = grep { /(.*)\Q$insuffix\E$/; $1 and not exists $outfiles{$1} } &GetINDirFiles($inpath);  

my $currenttime = time;        # get current time from system (epoch time)
@infiles = grep { -f "$inpath/$_" and ( $currenttime - (stat "$inpath/$_" )[9] ) > $timethreshold } @infiles;

# now you have all input files w/o corresponding output file that are older than 15 minutes
for (@infiles) {
		print "File $_ in $inpath directory was created ". ( $currenttime - (stat "$inpath/$_" )[9] )/60.0 ."minutes ago.\n";
		# put your action here
}