$_ = 'justify my love';
printf "[%20s] and [%-20s]\n", $_, $_;
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will give you

[     justify my love] and [justify my love     ]
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You can actually combine sprintf and formats, if you want to avoid specifying the width as a picture. You can use @* as your format for a line, and do all the actual formatting of the line with a sprintf. That assumes that you have some reason for using format/write instead of doing it all with printf.