There aren't a lot of use cases for storing an octal "number" in a scalar. There are some, but they're all to do with formatting output, not actual use as a number.

If your goal is to use this for chmod, you have to realise that it won't work. If you do this:

$mode = sprintf "%o", 420; # $mode = 644
chmod $mode, $file;
[download]

you'll actually be doing the same thing as this:

chmod 01204, $file; # 01204 octal == 644 decimal
[download]

Definitely not what you wanted. Leave the number as-is if you plan on using it in chmod. Note that the following lines are equivalent:

# 1. octal number
chmod 0644, $file;
# 2. decimal number
chmod 420, $file;
# 3. number in a scalar
$mode = 0644;
chmod $mode, $file;
# 4. number in a scalar (from decimal)
$mode = 420;
chmod $mode, $file;
# 5. number with bitmasks
$mode = 0666 & ~022; # like umask
chmod $mode, $file;
# 6. decimal with bitmasks
$mode = 438 & ~022; # 438 == 0666
chmod $mode, $file;
[download]

So, to answer your question, the number is already in the scalar. You don't actually need to do anything about it.

And, if you use $mode = sprintf "%o", $mode;, remember what you just did: you took a number, you printf'd it into a string, and put that back in $mode. So $mode no longer carries a number but a string. Perl will try to force it back to a number if that's what you try to do, but it won't be the number you started with.

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