Here's my solution, it also handles the more deeply nested cases like <r><p>a<x>b<y>c<x>d<s/>e</x>f</y>g</x>h</p></r>:

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

my $dom = XML::LibXML->load_xml(string => <<'END_XML');
<?xml version="1.0"?>
<office:document-content office:version="1.2"
    xmlns:office="urn:oasis:names:tc:opendocument:xmlns:office:1.0"
    xmlns:text="urn:oasis:names:tc:opendocument:xmlns:text:1.0">
<office:body><office:text>
<text:p text:style-name="P1">
    Fo<text:span text:style-name="T1">o<text:line-break/>B</text:span>
    <text:span text:style-name="T3">a</text:span>
    <text:span text:style-name="T5">r<text:line-break/></text:span>
</text:p>
</office:text></office:body>
</office:document-content>
END_XML

my $xpc = XML::LibXML::XPathContext->new($dom);
$xpc->registerNs('office',
    'urn:oasis:names:tc:opendocument:xmlns:office:1.0');
$xpc->registerNs('text',
    'urn:oasis:names:tc:opendocument:xmlns:text:1.0');
my $breakat = 'text:line-break';
my $ancestor = 'text:p';
while (1) {
    my ($br1) = $xpc->findnodes("//$ancestor//${breakat}[1]") or last;
    die "can't handle <$breakat> with children: $br1"
        if $br1->hasChildNodes;
    my ($an1) = $xpc->findnodes("ancestor::${ancestor}[1]",$br1)
        or die "failed to find <$ancestor> ancestor of $br1";
    my $an2 = $an1->cloneNode(1);
    my ($br2) = $xpc->findnodes(".//${breakat}[1]", $an2)
        or die "internal error: failed to find <$breakat> in $an2";
    for ( my $cur = $br1; $cur!=$an1 ; $cur = $cur->parentNode ) {
        while ( my $s = $cur->nextSibling )
            { $s->unbindNode }
    }
    $br1->unbindNode;
    for ( my $cur = $br2; $cur!=$an2 ; $cur = $cur->parentNode ) {
        while ( my $s = $cur->previousSibling )
            { $s->unbindNode }
    }
    $br2->unbindNode;
    $an1->parentNode->insertAfter($an2, $an1);
}
print $dom;

__END__

<?xml version="1.0"?>
<office:document-content xmlns:office="urn:oasis:names:tc:opendocument
+:xmlns:office:1.0" xmlns:text="urn:oasis:names:tc:opendocument:xmlns:
+text:1.0" office:version="1.2">
<office:body><office:text>
<text:p text:style-name="P1">
    Fo<text:span text:style-name="T1">o</text:span></text:p><text:p te
+xt:style-name="P1"><text:span text:style-name="T1">B</text:span>
    <text:span text:style-name="T3">a</text:span>
    <text:span text:style-name="T5">r</text:span></text:p><text:p text
+:style-name="P1"><text:span text:style-name="T5"/>
</text:p>
</office:text></office:body>
</office:document-content>

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In reply to Re^3: fix ODT files with line breaks looking poor by haukex
in thread fix ODT files with line breaks looking poor by melutovich

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