Is the tm element the only child of another node? If so, you can get the parent of the matching node, remove it's children, then insert the text node:

use strict;
use warnings;
use XML::LibXML;

my $xml_str = <<_XML_STR;
<foo>
  <bar>
    <tm tmclass="ibm" tmowner="IBM Corporation" 
        tmtype="reg" trademark="AIX">AIX</tm>
  </bar>
</foo>
_XML_STR

my $parser = XML::LibXML->new();
my $doc    = $parser->parse_string( $xml_str );
my $query  = "//tm";

foreach my $node ( $doc->findnodes($query) ) {
  my $val = $node->findvalue( '@tmowner' );
  my $txt = $node->textContent();

  if( $val !~ /my_company/ ) {
    my $parent = $node->parentNode();
       $parent->removeChildNodes();
       $parent->appendTextNode( $txt );
  }
}

print $doc->toString;
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<foo>
  <bar>AIX</bar>
</foo>
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