Hmm, that error should have motivated you to type

$ perldoc HTTP::Response
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and move on from the knowledge gained

my $res = $mech->mirror (...);
if ($res->is_success) {
    my $content = $res->decoded_content;
    # Do something with content
    }
else {
    warn "Oops: ", $res->status_line;
    }
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