From the CGI pod:

 popup_menu() creates a menu.

       1.  The required first argument is the menu's name
           (-name).

       2.  The required second argument (-values) is an array
           reference containing the list of menu items in the
           menu.  You can pass the method an anonymous array, as
           shown in the example, or a reference to a named array,
           such as "\@foo".

You need to pass a reference to a list to the -values key of a popup_menu:

popup_menu( -name => 'select', -values => \@files )
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Please read the CGI documentation carefully. It is well-written and has many examples.