I think you misunderstood the [, $level] part of the docs: it's intended to mean that the ", 1" part is optional. As you wrote it it simply won't compile.

syntax error at test.pl line 3, near "[,"
Execution of test.pl aborted due to compilation errors.
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Try this:

   use Compress::Zlib;
   my $str_to_compress='a big string';
   $compressed_str=compress($str_to_compress,1);
   $decompressed_str=uncompress($compressed_str);
   print $decompressed_str;
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By the way, please note exact error message text next time. Paraphrasing errors will only confuse people.