Another quick solution is to write to STDERR directly

=================================== 
#!/usr/bin/perl -w 
use strict; 
print STDERR "Begin...\n"; 
for ( my $i = 0; $i < 5; $i++ ) 
{ 
  print STDERR "$i...\n"; sleep 2; 
} ===================================
[download]

$ ./populate.pl 2> data.txt& (assumes bourne shell or bash)
STDERR is unbuffered, so you get the behavior you expect