UPDATE:As marto points out, the OP's question was the number of images in a PDF file, not the number of pages. Thus the below is not the answer asked for.

I have never used it, but quickly browsing the documentation you linked in your post I found that you can use the 'pages' function:

$pagenumber = $pdf->pages

    Returns the number of pages in the document.

For Example:

use PDF::API2;
my $pdf = PDF::API2->open('test_file.pdf');
print "'test_file.pdf' has " . $pdf->pages . " pages\n";
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