Perl: the Input Record Separator

Cited from slurp mode - reading a file in one step

The $/ variable is the Input Record Separator in Perl. When we put the read-line operator in scalar context, for example by assigning to a scalar variable $x = <$fh>, perl will read from the file up-to and including the Input Record Separator which is, by default, the new-line \n.

What we did here is we assigned undef to $/. So the read-line operator will read the file up-till the first time it encounters undef in the file. That never happens so it reads till the end of the file. This is what is called slurp mode, because of the sound the file makes when we read it.