Code expressions in regex refer to the regex group captured, by name / relative

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

How do our code evaluations in regex refer to the regex group captured, by name and/or relative way ? Try such

'foo' =~ /(?'name'(?=o)(\w+))(?{print $name})/

# or

'foo' =~ /(?'name'(?=o)(\w+))(?{print $-1})/

Number found where operator expected at (eval 23)
[download]

to only get failure.
How is the correct Perl's way ?

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