Yes, you're accessing the same $_ everywhere. So, by definition, it's a global.

Only, it temporarily gets a new value before the call to your sub, and it gets restored to its old value afterwards.

One way to that yourself is by using

local $_;
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or

local $_ = 'temporary value';
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which makes a copy of your value; or, as ikegami wrote in another reply, using for/foreach:

foreach('temporary value') {
    # your code
}
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which will loop exactly once, with $_ set to an alias of the value (meaning it's a different name for the same value, change the variable and the value will follow (or, for a constant, it might protest against the attempt to change a read-only value).