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    sub Open {
        my( $file, $opts )= @_;
        foreach $key (  %$opts  ) {
...
    }

    Open( "file", { ReadOnly=>1, Retries=>2 } );

    sub Open {
        my $opts= UNIVERSAL::isa( $_[-1], "HASH" )
            ? pop(@_) : {};
...
    Open( "file" );
    Open( "file", { Exclusive=> 1 } );
    Open( "file", "w", 0666, { Locked=>1 } );