If you're unfamiliar with *nix OSes, perhaps what I posted requires a little further explanation:
The actual code I ran is the "perl -Mstrict -Mwarnings -e ' ... '" part (see perlrun).
The two lines immediately following that second single quote is the output produced by the print statement.
cat is a commonly used *nix command (unrelated to Perl) that prints the contents of file(s).
You can read "$ cat pm_1045452_file1.txt" as "Here's the contents of the file pm_1045452_file1.txt:".
This is entirely unrelated to the Perl code; it merely shows the data the Perl code is using (which, as stated, I had slightly modified).
[In case you didn't know, "*nix" is just an umbrella term for any UNIX-like OS.]