#!/usr/bin/perl
use strict;
use warnings;

my $dir = $ARGV[1] || 'C:/devl/bin';
chdir $dir or die "Unable to change directories to $dir : $!";

my $now = time;
my $threshold = $ARGV[0] || 300;

opendir( DIR , '.' ) or die "Unable to read $dir : $!\n";
for ( readdir( DIR ) ) {
    if ( abs( $now - (stat($_))[9] ) > $threshold ) {
        Create_Warning_File();
        last;  # Unless you want 1 warning per file
    }
}

sub Create_Warning_File {
    # You did not give enough information to fill this in
}