The 1st "Use of uninitialized value in split" warning message you get is because the @url array does not have more that one element. Therefore, $url[1] is undefined. You can prove this to yourself by adding this debug line:

        @url = split( /Realtime koers $aandeel/, $html );
        print scalar(@url), "\n";  # show number of elements in array
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See also:

• Basic debugging checklist
• You should consider using an HTML parser.