I don't understand why you can't pass by value, but no matter, you can just dereference inside the sub. 

sub mysub{
    my $h = shift;
    my %dereferenced_hash = %$h;
    delete $dereferenced_hash{'b'};
}

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In reply to Re^3: Global variable unexpectedly modified when passed by reference by Anonymous Monk
in thread Global variable unexpectedly modified when passed by reference by fshrewsb

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