$arg = "$(mkdir otherdir1)";
Use single quotes in Perl, or the "$(" won't stick. And do something that produces output as well, to use in the name of the new directory. 
$arg = '$(mkdir otherdir1; date +%F_%H.%M.%S)';

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This also shows the need to cleanup (AKA "untaint") the user entered input data before just executing it. You could be executing a lot more than you bargained for.

In reply to Re^3: Is it possible to execute some command in qx argument by bart
in thread Is it possible to execute some command in qx argument by vladimirfedorov

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