That's what people call a 'slurp'. There's many ways to do it and if you search PerlMonks for 'slurp' you'll find them. Here's one I cooked up earlier:

# FILE is already open ...
{
   local $/ = undef;
   $slurp = <FILE>;
}
[download]

Much better (and faster) than using joins, or loops to append. Keep it inside the {} block so that the change to $/ is only temporary. (also see $/ at perldoc). The file's entire content ends up in $slurp.