mtime is the number of seconds since 1970 (at least on UNIX-ish systems).
You can compare it to time() to see how old the file is.

my $mtime = (stat $filename)[9];
my $age = time() - $mtime;

die "HEY! $filename was created in the future!\n"
    if $age < 0;

print "$filename is $age seconds old\n";
[download]