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my $twig = XML::Twig->new(
    twig_handlers   => { subfield => \&editHref, },
...
    $text =~ s/\Q$orig/(OSt)20/;
    $href->set_text($text);
}

sub editHref {
    my ( $twig, $href ) = @_;
    my $text = $href->text();
    $href->set_text('(OSt)20') if $text eq '(OSt)16';
}