#!/usr/bin/perl
use strict;
use warnings;

my $LOG;

open_log();
bla();
close_log();

sub open_log {
    my $logfile = 'logfile.txt';
    open LOG, '>>', $logfile or die("Can't open logfile: $!\n");
    $LOG = *LOG;
}
sub bla {
    fill_log();
}

sub fill_log {
  print $LOG "$_\n" for (1..100);
}

sub close_log {
    close $LOG or die $!;
}