mkdir does not give any standard output. In case of failure of mkdir you see the error output which is not returned by backtick (``) operators. You may have to redirect error output to standard output to capture it.

my $op = `mkdir winlog 2>&1`;

On my system the error message in case of pre-existence of directory is "cannot create directory `winlog': File exists" which will fail your regular expression. I think using the return codes is a better idea than using the error output. You can get the return code in $?

For all codes returned by the mkdir check the detailed manual of the command

Update: As pointed by corion and marto below mkdir of perl is the correct option. My answer is more concerned with the way of getting output rather than the approach.

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