my $log = '/home/tsec/prototype/logs/extractedlogs/cowrieresult.log';
open(my $data, '<', $log) or die "Can't open '$log' for reading: $!";
tester($data);
close  $data              or die "Can't close '$log': $!";

sub tester
{
    my ($fh) = @_;

    while (<$fh>) {
        ...
    }
    ...
}