Can you be bit elaborate on the sample that you have provided.

By "the sample", I presume you mean this one (reformatted for readability)?

perl -e"
    local $/ = '~'; 
    push @{ $h{lines } }, $_ while <>; 
" huge.file
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When you call readline (or use the <> operater as above), Perl determines how much to read from the file, by looking for a character (or sequence of characters), that match the current setting of the special variable $/ (also known as the $INPUT_RECORD_SEPARATOR (you'll have to scroll down aways to find it)). Normally, $/ defaults to being a newline. But...

What the sample above does is set the value of $/ = '~';. That means that readline will stop reading when it encounters a '~' in the input stream.

Ie. Instead of readline reading the whole 37 MB in as a single string; then spliting it into a big list; and then assigning that to the array on mass.

The code above, readlines just up to the first '~' character, pushes it onto the array; then loops (while), back to get the next chunk up to the next '~'.

Put another way. Setting $/ = '~';, has the effect of redefining a line, as a sequence of chars terminated by a '~'.

I hope one of those descriptions helps--ask again if it doesn't--because I know of no other langauge that has a standard library that allows you to do this. So when you first encounter it, it is definitely a bit weird.