Yet another way, set up defaults when you construct the hash,

sub routine {
    my $input_ref = {
        var1 => '',
        var2 => $VAR2,
        # . . .
        @_
    };
    my $var1      = $input_ref->{var1};
    my $var2      = $input_ref->{var2};
    # ...etc...
}
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That way your defaults are in the hash to start with, and can be overridden by anything the user wants to assign. Less logic to go wrong.