I can't tell what GetXMLValue does, but generally using a module such as XML::Twig is the smart way to handle XML parsing and manipulation problems. Consider:

#!/usr/bin/perl
use strict;
use warnings;
use XML::Twig;

my $t = XML::Twig->new(
    twig_roots               => {'CLIENT_INFO/FIELD' => \&convert},
    twig_print_outside_roots => 1
    );

$t->parse(*DATA);

sub convert {
    my ($t, $elt) = @_;
    my $txt = $elt->text();
    $elt->set_text('FINANCIAL SYSTEMS') if $txt =~ /FINANCE GROUP/i;
    $elt->print ();
}


__DATA__
<CLIENT_INFO> <LOCATION> New York </LOCATION> <FIELD> FINANCE GROUP </
+FIELD> <NO_OF_WORKERS> 123 </NO_OF_WORKERS> </CLIENT_INFO>
[download]

Prints:

<CLIENT_INFO> <LOCATION> New York </LOCATION> <FIELD>FINANCIAL SYSTEMS
+</FIELD> <NO_OF_WORKERS> 123 </NO_OF_WORKERS> </CLIENT_INFO>
[download]