Autovivification happens in an lvalue context (a place where a variable might be modified, not just 'read'). Arguments to user subroutines can be modified (but usually aren't). Perl knows that the join built-in doesn't modify the arguments passed to it. Perl 5 does not provide a way for you to declare that a user subroutine refuses to modify any arguments, nor does Perl try to analyze subroutines to determine this and thus change how arguments are prepared to be passed in.

So don't put your slice into an lvalue context. Many ways to do that, including:

myjoin( @_= @hash{@keys} );
myjoin( my @x= @hash{@keys} );
myjoin( @{[ @hash{@keys} ]} );
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myjoin( map $_, @hash{@keys} );
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myjoin( map $hash{$_}, @keys );
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