You have some common mistakes in your script:
You are matching greedy - add a ? after your +, like this:

$riga =~ s/<\/.+?>//g;
[download]

Also, you assume that the opening bracket and the closing are on the same line, which isn't usually neeeded. So adding the s modifier to your substitutions would help, too. And, you forgot to substitute > with > :)

There is lot of space for optimizing it, like using different delimeters to avoid having to escape the slash, or doing more substitutions just in one line, like the first two:

$riga =~ s!</?.>!!g;
[download]

I hope you get what I mean, nice script anyway.
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use signature; signature(" So long\nAlfie");