$_="I want to replace <P> in my string. I have to search for the <P> a
+nd replace it with space. Thanks";
s/<P>/ /gi;
print;
[download]

results in:

I want to replace   in my string. I have to search for the   and repla
+ce it with space. Thanks
[download]

update: You might also want to try perldoc perlre which explains regular expressions fairly well.