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expr := value ( op value )*

sub parse_expr {
    my($value, @stack);
...
    $value = process(\@stack, $value);  # final processing
    return $value;
}

value := '(' expr ')'

#!perl -w
use strict;
...
$_ = "20+3*a+10*-b-5*(3 +2)*5";
$\ = "\n";
print evaluate($_);

#!perl -w
use strict;
...
$_ = "20+3*a+10*-b-5*(3 +2)*5";
$\ = "\n";
print evaluate($_);

value := name '(' ( expr ( ',' expr ) * )? ')'