pjotrik is right, the example was not XML. If it were, this is a solution using XML::Rules (AZed, neither this solution nor XML::Twig has any memory problems whatsoever with this. Not all XML parsers construct a maze of objects before you can get your hands on the data.)

#!/usr/bin/perl
use strict;
use warnings;

use XML::Rules;

print "This will calculate the total amount and To no of transactions 
+\n";

my $totalamt = 0;
my $cnt = 0;

my $parser = XML::Rules->new( stripspaces => 7,
    rules => {
        _default => '',
        b => sub {++$cnt;return},
        payment_amount => sub {$totalamt+=$_[1]->{_content};return},
    });
$parser->parse(\*DATA);

print "The total amount found is $totalamt \n";
print "Total Transactions are    $cnt      \n";

__DATA__
<a>
    <b>
            <client_account_cred>68789790390909090489</client_account_
+cred>
            <Payment_UTR_No>MTRIN10909890896</Payment_UTR_No>
            <payment_amount>700000</payment_amount>
    </b>
    <b>
            <client_account_cred>9033753053985392INR</client_account_c
+red>
            <Payment_UTR_No>938573895735154</Payment_UTR_No>
            <payment_amount>1222706</payment_amount>
    </b>
    <b>
            <client_account_cred>9284723472047222INR</client_account_c
+red>
            <Payment_UTR_No>RP JLLKL7687</Payment_UTR_No>
            <payment_amount>1437865.95</payment_amount>
    </b>
</a>
[download]

or if you do not want to use file scoped lexicals (and want the parser to be able to give you the right answer even if you call it several times):

#!/usr/bin/perl
use strict;
use warnings;

use XML::Rules;

print "This will calculate the total amount and To no of transactions 
+\n";


my $parser = XML::Rules->new( stripspaces => 7,
    start_rules => {
        a => sub {
            $_[4]->{pad}{count}=0;
            $_[4]->{pad}{total}=0;
            1;
        }
    },
    rules => {
        _default => '',
        b => sub {++ $_[4]->{pad}{count}; return},
        payment_amount => sub {$_[4]->{pad}{total} += $_[1]->{_content
+};return},
        a => sub {return %{$_[4]->{pad}}}
    });
my $data = $parser->parse(\*DATA);

print "The total amount found is $data->{total}\n";
print "Total Transactions are    $data->{count}\n";

__DATA__
<a>
    <b>
...
[download]