If you just want to process the file periodically when it has changed, you can:

my $old_time = (stat( $file ))[9];

while ( 1 ) {
    sleep $period_in_seconds;
    my $current_time = (stat($file))[9];
    while ( $current_time != $old_time ) {
         $old_time = $current_time;
          print "file changed$/";
          $current_time = (stat($file))[9];
    }
}
[download]

The above nods at the issue of race conditions; it ignores possible contention issues.

Be well,
rir