my $slurp;
{
local $/ = undef;
open my $textfile, '<', 'filename.txt' or die $!;
$slurp = <$textfile>;
close $textfile;
}

##</code><code>##

open my $textfile, '<', 'filename.txt' or die $!;
while ( my $line = <$textfile> )
{
  ## Do something with $line
}