# at the start of cgi script

my $output = ''; # to store the output.

open SECOUT, ">&STDOUT";
tie *STDOUT, 'BLAH', $output;

....
......
....

# this is the last thing done before the script exits.

open OUTFILE, ">>test.log" or die "Cannot open blah blah.. $!\n";
print OUTFILE "----\n", $output;
close OUTFILE;

print SECOUT $output; # reprint the output to real STDOUT

...
....
..

# somewhere else.

package BLAH;

use Tie::Handle;
my $data;
sub TIEHANDLE {
	return bless {}, shift;
}

sub PRINT {
	my $self = shift;
	$data .= join '', @_;
}