do is "not really a function". It is a block that has a seperate scope from wherever it was called from. This allows you to localize $/ so that when execution leaves our "temporary" scope, $/ will be what it was before it was changed.

If you look up $/ in perlvar, you will see 

   You may set it to a multi-character
   string to match a multi-character terminator, or
   to "undef" to read through the end of file.
Finally, <DATA> returns the next line from the filehandle DATA, and since $/ is undefined, the entire contents of the file are returned and "caught" in a scalar.

UPDATE: i should add that this is one of those few cases where local is a good choice. For example, 
local $/;

[download]
Is essentially the same as 
local $/ = undef;

[download]
Which is not the same as 
$/ = undef;

[download]
Even though that last snippet will be contained inside a do block ... $/ will still be undef afterwards. Try this - make two files foo.txt and bar.txt and run this: 
# print entire file
open FH,'foo.txt';
my $foo = do {$/ = undef;<FH>};
print $foo;

# print only first line
open FH,'bar.txt';
my $bar = <FH>;
print $bar;

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Then go back and add local to the do block.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to Re: slurping __DATA__ by jeffa
in thread slurping __DATA__ by LanceDeeply

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