If you only have one file to monitor and nothing else to do while you wait for it to be updated, you might find File::Tail helpful.

update: Or, you could do something like the following:

use strict;
use warnings;

open(my $fh, '<', 'test.dat') or die "test.dat: $!";

while(1) {
    my $line = <$fh>;
    if(defined($line)) {
        print "$line";
    } else {
        sleep(10);
    }
}
close($fh);
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