You might want to split the string using Text::Balanced's extract_multiple function and then apply a map to extract your data (this code extracts the first '+' and replaces it with a '-'):

use strict;
use warnings;
use List::Util qw/first/;
use Text::Balanced qw/extract_multiple extract_bracketed/;
my $text    = "1*(2+3)*(3+4)+5*(6+7)+42";
my @results = extract_multiple(
    $text,
    [
        {
            Bracketed => sub { extract_bracketed( $_[0], '()' ) }
        },
        { PlusOperator     => qr{[+]} },
        { MultiplyOperator => qr{[*]} },
        { Number           => qr{\d+(?:\.\d+)?} },
    ]
);

my $operator = first { ref($_) eq 'PlusOperator' } @results;

# $operator holds '+'
my $plus;
my $new_string = join(
    '',
    map {
        if ( !$plus && ref $_ eq 'PlusOperator' ) {
            $plus = 1;
            '-'
        } else {
            $$_
        }
      } @results
);
print "$new_string\n";
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or the answer is 'no', regexes that parse data like that are never going to be elegant.