#!/usr/bin/perl
use strict;
use warnings;
use Fcntl qw/:seek/;

my $file='my.log';

# position to end
open(FILE,$file) || die "$file: $!";
seek(FILE,0,SEEK_END) || die "Seek: $!";
my $pos=tell(FILE);
close FILE;

while(1) {
   sleep(5*60);   # every 5 minutes
   open(FILE,$file) || die "$file: $!";
   seek(FILE,$pos,SEEK_SET) || die "Seek: $!";
   while (<FILE>) {
      #  do something with $_ lines
   }
   $pos=tell(FILE);   # update
   close FILE;
}