He's using <> so the original text file is implicitly opened if given on the cmd line. I guess you already knew, but you may have just overlooked it.

To be sure, just take his code and modify it like thus:

#!/usr/bin/perl -l

my @file = <>;
print 0+@file;
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Indeed

$ ./bar.pl data.txt 
46
$ wc data.txt 
 46  36 346 data.txt
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