It works by concatenating (with ".") the contents of $_ (your current line in this case) with the last line(s) read in. It is equivalent to: $_ = $_ . <YOURFILE>. You can have a look at perldoc perlop under "Assignment Operators" for a more detailed explanation.

If you have:

line one
line two 
line three
[download]

the first time around, $_ is "line one\n", the second time around, $_ becomes "line one\nline two\n", and the third time around, $_ becomes "line one\nline two\nline three\n", which allows you to use your regex across more than one line in the file.

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Allolex