That should be

$str="himanshu.txt^@";
if($str =~ /(\w+)\.(\w+)/) {
    $str = "$1.$2";
}
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What is your "^@" ? That looks like an ASCII 0, as less(1) would show it. You can sense that and remove it with

if ($str =~ s/\0$//) {
    print "found NUL byte at the end of '$str'\n";
}
[download]