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expr      := number
           | expr ws unary_operator 
...
ws        := ' ' | '\t' | '\r' | '\n'

Note: I use "chs" as shorthand for 'c' 'h' 's'

digit  -> /(?<digit> \d )/msx
number -> /(?<number> (?&digit)+ (?:[.](?&digit)*)
                |     [.](?&digit)+ )/msx

qr{
(?(DEFINE)
...
)
    \A \s* (?&expr) \s* \z
}msx;

String found where operator expected at (re_eval 10) line 1, near "say
+ "matching at ""
    (Do you need to predeclare say?)
syntax error at (re_eval 10) line 1, near "say "matching at ""
Compilation failed in regexp at rpg.pl line 137.

(?(DEFINE)
    (?<expr>
...
    \A \s* (?&expr) \s* \z
    (?(?{ $^R != 1 }) (*FAIL) ) 
}msx

  /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
      (?(DEFINE)
          (?<NAME_PAT>....)
          (?<ADRESS_PAT>....)
      )/x

    (?<number>
        ( (?> \d+ (?: [.] \d* )? | [.] \d+ ) )
        (?{ local @stack = ( @stack, $^N ) })
    )

qr{
(?(DEFINE)
...
    (?{ $result = undef }) (*FAIL)

}msx

qr{
    (?{ @stack  = ();
...
    (?(?{ @stack != 1 }) (*FAIL) )
    (?{ $result = $stack[0] })
}msx,

my $input = join ' ', (1) x 1_001, ('+') x 1_000;

...
            s/iter recursive      flat
recursive     1.35        --      -99%
iterative 1.47e-02     9103%        --

# my $input = join ' ', 1, ('1 +') x 1_000;
            Rate recursive iterative
recursive 40.5/s        --      -40%
iterative 67.6/s       67%        --

# my $input = join ' ', (1) x 4, ('+') x 3;
             Rate recursive iterative
recursive 14306/s        --      -17%
iterative 17271/s       21%        --

qr{
    \A \s*
...
    \s* \z
    (?(?{ $^R != 1 }) (*FAIL) )
}msx,

# my $input = join ' ', (1) x 1_001, ('+') x 1_000;
            Rate recursive iterative
...
             Rate recursive iterative
recursive 28249/s        --      -69%
iterative 92593/s      228%        --

#!//net/perl/5.10.0/bin/perl
use warnings;
...
        say "FAILED: $line got $result, expected $input{$line}";
    }
}