Is an 'honorary' built-in something that is very common but not part of Perl language?

It's a bit more restricted than that. Honorary built-ins are subroutines from core Perl modules that provide functionality that ought to be in the language itself -- for example, carp and croak from the core Perl Carp module.

Surely prototypes, in some circumstances, are extremely useful

Rarely. There are many references on this tricky topic.

Subroutine Prototype References

• Far More than Everything You've Ever Wanted to Know about Prototypes in Perl -- by Tom Christiansen - classic article posted at Perl Monks by liverpole
• Gratuitous use of Perl Prototypes by grantm (2004)
• Perl Best Practices item 122 "Don't use subroutine prototypes"
• Perl::Critic::Policy::Community::Prototypes - Don't use function prototypes

• Understand why you probably don't need prototypes by brian_d_foy (The Effective Perler)
• Use v5.20 subroutine signatures by brian_d_foy (The Effective Perler)
• Filter::signatures by Corion - very simplistic signatures for Perl < 5.20 (see Re: features and backwards compatibility)

• Guacamole - a parser toolkit for Standard Perl

• Fun with Prototypes by haukex (2017)
• Re: Perl script compressor by haukex (2019)

• Too many arguments for subroutine by anonymonk (2023)

perldoc References

• Prototypes - a very limited kind of compile-time argument checking using function prototyping
• Named Unary Operators - named unary operators are treated as functions with one argument, with optional parentheses
• Signatures - enabled automatically by a use v5.36 or higher declaration, or more directly by use feature 'signatures' in the current scope

Updated: Added many references long after the original node was written.