The best advice is to use one of the many XML parsing modules.

That said, your approach does work for the sample data you have provided. I can't resist offering you your same approach with a couple of minor improvements in the hope that you'll find them helpful.

print "This will calculate the total amount and To no of transactions 
+\n";
my ($cnt, $totalamt);

# precompiled regex
my $PAYMENT_AMOUNT_PATTERN = qr{
    <payment \s+ amount>  # opening tag
    (                     # begin capture
        \d+[.]?\d*        # digits with optional decimal point
    )                     # end capture
    </payment \s+ amount> # closing tag
}msx;

LINE:
while (<DATA>) {
    # capture to lexical variable
    my ($payment_amount) = /$PAYMENT_AMOUNT_PATTERN/;
    next LINE if not defined $payment_amount;
                
    $cnt++;
    $totalamt += $payment_amount;
    print;
}
print "The total amount found is $totalamt \n";
print "Total Transactions are    $cnt      \n";
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