@ is a reference operator. The code you have written won't even compile I guess.
Since you are using double quotes to enclose your string, double quote will be the escape sequence for your strings. try using a double quote before the @.
I mean the following:
But I tried the following code: It works perfectly as expected.
</font><blockquote><font size="1" face="Verdana, Arial, Helvetica">code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">[-] testcase try_refer() appstate none
[ ] string sChange = strTran("HelloHowAreYou.", ".", "@"
[ ] print(sChange)</pre><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">The result was HelloHowAreYou@