Since Perl 5.8.0 you can open a filehandle for writing to a scalar. Then, if you make that filehandle the default, writes will go there.

my %hash = (one => 1, two => 2, three => 3);

my $output;
open FH, '>', \$output;
my $old_fh = select(FH);

my $key;
foreach $key (keys %hash) { 
    write;
}   

select ($old_fh);

print $output;

format FH =
@<<<<<<<<< @##
$key, $hash{$key}
.
[download]

Content of $output:

three        3
one          1
two          2
[download]

Unfortunately, this doesn't seem to work with localized filehandles.