sub write_stuff {
    open my $fh, '>>', $logfilename or die "Can't append to '$logfilename': $!";
    ...
    check_stuff();
    ...
};

sub check_stuff {
    open my $fh, '<', $logfilename or die "Can't read '$logfilename': $!";
    while (<$fh>) {
        ...
    };
};