I personally believe that you should explain exactly what you couldn't understand, given that you've been shown the relevant documentation:

my $pro = first { $user[0] eq $_ } @protected;
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assigns to $pro the first value in @protected which is equal to $user[0], that is, $user[0] itself, if the latter is in @protected, or else an undefined value. Thus the following line chooses what to do depending on whether $pro is defined or not, which appears to be what you want, ain't it?