#!/usr/bin/perl 
use warnings;
use strict;
use IO::Handle;

open (my $fh, '>', 'logfile.txt')  || die "can't open logfile.txt";
open (STDERR, ">>&=", $fh)         || die "can't redirect STDERR";
$fh->autoflush(1);

my $need_work = 5;
my $i_tried   = 0;

while ( $need_work > $i_tried ) 
{
    my $a;
    $i_tried++;
 
    print "$a\n";   #this generates a run time warning!!!!!!
    warn "this is warning $i_tried"; #an explict warning 
    print $fh "I've tried $i_tried things as a test\n";
}
__END__
logfile.txt contains:

Use of uninitialized value $a in concatenation (.) or string at C:\TEMP\junk2.pl line 18.
this is warning 1 at C:\TEMP\junk2.pl line 19.
I've tried 1 things as a test
Use of uninitialized value $a in concatenation (.) or string at C:\TEMP\junk2.pl line 18.
this is warning 2 at C:\TEMP\junk2.pl line 19.
I've tried 2 things as a test
Use of uninitialized value $a in concatenation (.) or string at C:\TEMP\junk2.pl line 18.
this is warning 3 at C:\TEMP\junk2.pl line 19.
I've tried 3 things as a test
Use of uninitialized value $a in concatenation (.) or string at C:\TEMP\junk2.pl line 18.
this is warning 4 at C:\TEMP\junk2.pl line 19.
I've tried 4 things as a test
Use of uninitialized value $a in concatenation (.) or string at C:\TEMP\junk2.pl line 18.
this is warning 5 at C:\TEMP\junk2.pl line 19.
I've tried 5 things as a test