The error you were getting was, Can't modify non-lvalue subroutine at powerpoint_create_withEDM.pl line 259. Your problem point was this,

$pres -> Slides($slideIndex) = $slides[$k];
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Generally, in Perl, you cannot do an assignment of a value to a function/method. A lvalue assignment would like like this,

$foo->bar() = 'my value';
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I know that traveler solved your problem but maybe this will help you when you try and debug the next issue...