Here is an example that uses Pod::Select. It doesn't preserve the =cut directives but that probably isn't a big concern.

Since you wanted to have the output in a string this example uses perl 5.8's really useful feature of opening a scalar as a filehandle. For older perls you could use IO::Scalar instead.

#!/usr/bin/perl -w

use strict;
use Pod::Select;

# perl 5.8 feature, open a scalar as a filehandle
open my $fh_out, '>', \my $pod or die "Couldn't open filehandle: $!";

my $fh_in = \*DATA;

my $parser = Pod::Select->new();
   $parser->parse_from_filehandle($fh_in, $fh_out);


print $pod;


__DATA__

# Some code
my $foo = 'bar';

=head1 This is a B<heading>

This is a paragraph.

    This is a verbatim section.

This is I<B<another>> paragraph

=cut
[download]

--
John.