use strict;
use warnings;

use XML::Twig;

my %self2pnam; # Self => pnam for all color elements

my $twig=XML::Twig->new(                 # create self2pnam
                        twig_handlers=>{ color => sub { $self2pnam{$_->att('Self')}= $_->att( 'pnam'); } },
                        pretty_print => 'indented',
                        );
$twig->parse(\*DATA);

# now we can process the content elements
foreach my $content ($twig->root->children( 'content'))
  { my $box= $content->prev_elt( 'box[@type="text"]');
    $content->set_tag( 'updated_tag')
            ->del_atts
            ->set_att( Color => $self2pnam{$box->att( 'frameColorID')})
            ;
  }

$twig->print;


__DATA__
<document>
  <root>
  <box type="text" cont="text" textflowid="rc_u5035"  frameColorID="ua5"/>
  </root>
  <content type="text" Self="rc_u5035" id="rc_u5035"/>
  <header>h3</header>
  <meta>
  <color type="colr" clmd="prss" clsp="CMYK" clvl="4_D_0_D_15_D_50_D_0" ovrd="norm" atcs="nasp" atvl="0" pnam="Color 04-15M/50Y" edbl="t" rmbl="t" pvis="t" swID="1f01" Self="uce"/>
  <color type="colr" clmd="prss" clsp="CMYK" clvl="4_D_0_D_10_D_40_D_0" ovrd="norm" atcs="nasp" atvl="0" pnam="Color 05-10M/40Y" edbl="t" rmbl="t" pvis="t" swID="1f01" Self="ua5"/>
  </meta>
</document>