#!/usr/bin/perl
use strict;
use warnings;

my $line = '~1~,,~234~,7,~,,~,8';

my $newline = ""; # löl
my $in = 0;
for my $i ( 0 .. length($line) ) {
        my $c = substr($line, $i, 1);
        $newline .= $c;
        print $newline;
        
        # determine whenever we are in a field or outside.
        if( $in == 0 and $c eq '~' ) {
                # now we're in a field
                $in = 1;
                next;
        }if( $in == 1 and $c eq '~' ) {
                # now we're going outside to play hide and ...
                $in = 0;
                next;
        }if( $in == 0 and $c eq ',' ) {
                # we're outside, now check if next char is a , too
                if( substr($line, $i, 2) eq ',,' ) {
                    print ' -> ' . substr($line, $i, 2);
                    $newline .= '~~';
                }
        }
        
        print "\n";
}

print( $newline );