From the docs:

# get some page contents
my $page = $mw->get_page( { title => 'Main Page' } );
# print page contents
print $page->{'*'};
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So you get at the pages's content by accessing the '*' key of the hash reference returned by the MediaWiki::API object's get_page() method. That will be HTML so you then need to parse it to get at the 'plain' text. There are many ways to extract the text from the HTML (see CPAN) depending on the structure you want to end up with - if you can tell us a little bit more we can point you further.