join can take arbitrary lists, whereas . (dot) concatenates the left hand side with the right hand side.

sub concat {
    return join '', @_;
}
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Or if you only want to work with two args, there is the dot operator:

sub concat {
    return $_[0] . $_[1];
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Interpolation also works:

sub concat {
    local $" = '';
    return "@_";
}
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Unless you can think of a good reason otherwise, the first solution is probably the most clear and flexible -- a more generalized solution.

I think Perl was given the smallest possible operator (dot) for concatenation of strings to make the C people feel bad.