https://metacpan.org/pod/HOP::Stream#promise says

promise


  my $promise = promise { ... };

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A utility function with a code prototype (sub promise(&);) allowing one to specify a coderef with curly braces and omit the sub keyword.

For debugging you might try https://metacpan.org/pod/HOP::Stream#show

In reply to Re: How to understand chapter 6 of Higher Order Perl? (promise/show) by Anonymous Monk
in thread How to understand chapter 6 of Higher Order Perl? by Anonymous Monk

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