You can error-check mkdir much like you do open:

mkdir $newdir or die "Unable to mkdir $newdir : $!";
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That's true of most commands in Perl; they return true if successful and false on failure, so you can use "do_this() or show_error()" logic. (One exception is system, which returns the return value of the underlying command, which (on Unix at least) is zero on success, so you have to watch out for that.)