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";
}