#!/usr/bin/perl

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

my $xmlstring;
{
    local $/;
    $xmlstring = <DATA>;
}
my $xml = XML::LibXML->new;
my $doc = $xml->parse_string( $xmlstring );
   # you can do $xml->parse_file( "filename" ) instead

for my $font_node ( $doc->findnodes( "//font" )) {
    my %attr_val;
    my $matched_text_nodes = 1;
    my $matched_content;
    my @text_nodes = $font_node->findnodes( "./text" );
    for my $tnode ( @text_nodes ) {
        my @atts = $tnode->attributes;
        my $y_indx = grep { $atts[$_]->nodeName eq 'y' } 0 .. $#atts;
        my $p_indx = grep { $atts[$_]->nodeName eq 'page' } 0 .. $#atts;

        if ( ! keys %attr_val ) {  # first text_node
            $attr_val{y} = $atts[$y_indx]->textContent;
            $attr_val{p} = $atts[$p_indx]->textContent;
        }
        elsif ( $attr_val{y} ne $atts[$y_indx]->textContent or
                $attr_val{p} ne $atts[$p_indx]->textContent ) {
            $matched_text_nodes = 0;
        }
        if ( $matched_text_nodes ) {
            $matched_content .= $tnode->textContent . " ";
        }
    }
    if ( $matched_text_nodes ) {
        $text_nodes[0]->firstChild->setData( $matched_content );
        $font_node->removeChild( $_ ) for ( @text_nodes[1..$#text_nodes] );
    }
}
print $doc->toString;

__DATA__
<doc>
<font size="12" face="IJCINN+AvantGarde-Bold" color="#1BADEB">
<text x="198" y="200" width="32" height="12" page="vii">Part I</text>
<text x="242" y="200" width="75" height="12" page="vii">Introduction</text>
<text x="329" y="200" width="7" height="12" page="vii">2</text>
</font>
<font size="9" face="IJCINN+AvantGarde-Bold" color="#231F20">
<text x="183" y="221" width="47" height="9" page="vii">Chapter 1</text>
</font>
<font size="10" face="IJCIOP+Frutiger-Light" color="#231F20">
<text x="242" y="220" width="121" height="10" page="vii">Managers and Management</text>
<text x="373" y="220" width="6" height="10" page="vii">2</text>
</font>
<font size="9" face="IJCINN+AvantGarde-Bold" color="#231F20">
<text x="198" y="234" width="32" height="9" page="vii">History</text>
<text x="195" y="246" width="35" height="9" page="vii">Module</text>
</font>
<font size="12" face="IJCINN+AvantGarde-Bold" color="#1BADEB">
<text x="194" y="292" width="36" height="12" page="vii">Part II</text>
<text x="242" y="292" width="54" height="12" page="vii">Planning</text>
<text x="308" y="292" width="15" height="12" page="vii">56</text>
</font>
</doc>