sub transform {
   my ($data,$log_fh) = @_;
	
   # Localize the log filehandle so all subroutines can see it
   local *LOG_FH = $log_fh;

   ...
   foo($bar);
}

sub foo {
   my ($bar) = @_;
   print LOG_FH "Processing $bar.\n";
   ...
}

##</code><code>##

sub transform {
   my $data = shift;
   local $log_fh = shift;

   ...
   foo($bar);
}

sub foo {
   my ($bar) = @_;
   print $log_fh "Processing $bar.\n";
   ...
}

##</code><code>##

our $log_fh;
sub transform {
   my $data = shift;
	print $log_fh "Processing data $data.\n";
   foo("bar");
}

sub foo {
   my ($bar) = @_;
   print $log_fh "Processing $bar.\n";
}

sub main {
	open ( local $log_fh, '>', 'test.txt' ) or die "Couldn't open log file\n";
	transform( "data", $log_fh );
}
main();