Problem #1: You ask for the foo nodes that are children of the document. There are no such nodes.

$doc->findnodes('foo')
[download]

$doc->findnodes('seg/foo')
[download]

$doc->findnodes('/seg/foo')
[download]

Problem #2: You ask for the text children of the foo element, but it doesn't have any. It's not even text you want!

$foo->findnodes('text()')
[download]

join('', map { $_->toString() } $foo_node->childNodes)
[download]

join('', $foo_node->childNodes)
[download]

Also,

$foo->findvalue('@mid')
[download]

is better written as

$foo->getAttribute('mid')
[download]

So, we get

use strict;
use warnings qw( all );
use feature qw( say );

use XML::LibXML qw( );

my $xml = <<'__EOS__';
<seg><foo mid="0" mtype="seg"><g id="1">Need to export this text</g></
+foo></seg>
__EOS__

my $doc = XML::LibXML->new->parse_string($xml);
for my $foo_node ($doc->findnodes('/seg/foo')) {
   my $mid = $foo_node->getAttribute('mid');
   my $inner_xml = join('', $foo_node->childNodes);
   say "$mid $inner_xml";
}
[download]