@_ contaisn aliases to the parameters you pass, so if you work directly on $_[0], yo ucan change its value. It's the

my $t = shift;
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that makes a copy.

There are several ways to make a variable alias to an existing value, but, shame on Perl, none are quite what I'd call "easy". For example:

    *t = \($_[0]);
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which uses a global variable $t. With local you can restrict the damage, but it'll still not pass strict — you still need to declare $t – but not using "my"!

another way is using for in a non-obious way (for can make a loop variable that is an alias for each item it iterates over, and with a scalar it will loop exactly one):

for my $t ($_[0]) {
    # ...  the rest of the sub body
}
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There is even a few Alias modules on CPAN, the most interesting one always looked to me to be Data::Alias but I must confess I've never used it. (I tried to, long time ago, but it wouldn't compile back then.) Actually, looking at it now, I think Lexical::Alias might be more interesting — if it works.