Try this. It takes 3 seconds to update a generated file containing 1000 books (2000 titles):

#! perl -slw
use strict;
use Data::Dump qw[ pp ];
use XML::Simple;

my $start = time;

my $xml = XMLin( \*STDIN, KeepRoot => 1 );
#pp $xml;

for my $book ( 0 .. $#{ $xml->{catalog}{book} } ) {
    my $titles = $xml->{catalog}{book}[ $book ]{ titles }{ title };
    for my $title ( 0 .. $#{ $titles } ) {
        my $txt = $titles->[ $title ];
        $txt =~ s[(\d+)]{ $1+1 }e;
        $titles->[ $title ] = $txt;
    }
}

print XMLout( $xml, KeepRoot => 1, NoAttr => 1  );

warn time() - $start;
__END__
C:\test>junk < junk.dat >junk2.dat
3 at C:\test\junk.pl line 22.
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