my %function = (
  sumsq => sub  { my $sum = 0; foreach(@_) { $sum += $_*$_; } return $sum; },  # sum of squares
  sqrt => sub { return sqrt shift; },
);

##</code><code>##

    if(/\G((?i:[a-z]\w*))\s*\(/gc) {  # function '('
        my $function = $1;
        $function{$function} or die sprintf "Undefined function '$function' called at: \"%s\"", where();
        my @arg;
        unless(/\G\s*(?=\))/gc) {
            while(1){
                my($value) = parse_expr() or die sprintf "Expression expected at: \"%s\"", where();
                push @arg, $value;
                /\G\s*,/gc or last;
            }
        }
        /\G\s+/gc;
        /\G\)/gc or die sprintf "Parse error: ')' expected at: \"%s\"", where();
        trace(sprintf "function '$function' called with %d argument%s", scalar @arg, @arg==1 ? "" : "s");
        return $function{$function}->(@arg);
    }

##</code><code>##

Line 29 "·sumsq(3,2+2)*sqrt(36)"
Line 29 "sumsq(·3,2+2)*sqrt(36)"
Line 31 "sumsq(3·,2+2)*sqrt(36)" value=3
Line 29 "sumsq(3,·2+2)*sqrt(36)"
Line 31 "sumsq(3,2·+2)*sqrt(36)" value=2
Line 36 "sumsq(3,2+·2)*sqrt(36)" op=+
Line 29 "sumsq(3,2+·2)*sqrt(36)"
Line 31 "sumsq(3,2+2·)*sqrt(36)" value=2
Line 43 "sumsq(3,2+2·)*sqrt(36)" popping 2 +
Line 45 "sumsq(3,2+2·)*sqrt(36)" result = 4
Line 85 "sumsq(3,2+2)·*sqrt(36)" function 'sumsq' called with 2 arguments
Line 31 "sumsq(3,2+2)·*sqrt(36)" value=25
Line 36 "sumsq(3,2+2)*·sqrt(36)" op=*
Line 29 "sumsq(3,2+2)*·sqrt(36)"
Line 29 "sumsq(3,2+2)*sqrt(·36)"
Line 31 "sumsq(3,2+2)*sqrt(36·)" value=36
Line 85 "sumsq(3,2+2)*sqrt(36)·" function 'sqrt' called with 1 argument
Line 31 "sumsq(3,2+2)*sqrt(36)·" value=6
Line 43 "sumsq(3,2+2)*sqrt(36)·" popping 25 *
Line 45 "sumsq(3,2+2)*sqrt(36)·" result = 150
150
Stack: This value is never affected