use strict;
use warnings;

use IO::Handle;

my $log_file = 'mylogfile.txt';

open my $log, '>>', $log_file
   or die "Unable to open log file \"$log_file\": $!\n";

STDOUT->autoflush(1);
$log->autoflush(1);

my $foo=0;
while(1){
   $foo++;
   printf "Iteration: %d\n",$foo;
   $log->printf("Looped in %d\n",$foo);  # or printf $log ...
   sleep 2;
}