or 'eval' the use: 
eval('use dir_'.$day.'::'.$userId.' qw(\%diary)');
print "Can't use dir_$day::$userId module: $@";

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If you choose to use require, beware that if you give it a string, it expects it to be a file name/path, not the module name.

In reply to Re: How to use variables in @INC and "use" commands by pjotrik
in thread How to use variables in @INC and "use" commands by Arun Tv

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