if(defined($_[0])) {
            ...
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You forgot to handle the case when $_[0] is undef, i.e. when the form open my $fh, ... is being used.

BEGIN {
    *CORE::GLOBAL::open = sub (*;$@) {

        use Symbol ();
        my $handle = Symbol::qualify_to_ref($_[0], scalar caller);

        $_[0] = $handle unless defined $_[0];  # pass up to caller

        if (@_ == 1) {
            CORE::open $handle;
        } elsif (@_ == 2) {
            CORE::open $handle, $_[1];
        } elsif (@_ == 3) {
            if (defined $_[2]) {
                CORE::open $handle, $_[1], $_[2];
            } else {
                CORE::open $handle, $_[1], undef;  # special case
            }
        } else {
            CORE::open $handle, $_[1], $_[2], @_[3..$#_];
        }
    };               
}

# sample usage:

open my $fh, ">", \my $buf or die $!;
print $fh "hello";

print $buf;   # prints "hello"
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