This test indicates the data is still available for additional reading.

use strict;
use warnings;

my $data;
my $bytes = read( DATA, $data, 10 );
print "read $bytes bytes: [$data]\n"; # read 10 bytes: [1234567890]
$bytes = read( DATA, $data, 5 );
print "read $bytes bytes: [$data]\n"; # read 5 bytes: [abcde]

__DATA__
1234567890abcdefg
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Update: As mentioned in the docs, read returns the number of bytes actually read, 0 at end of file, or undef if there was an error. This means you can loop until read returns false:

use strict;
use warnings;

my $data;
while( read( DATA, $data, 5 ) )
{
    print "[$data]\n";
}

__DATA__
Just another Perl hacker,
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This prints

[Just ]
[anoth]
[er Pe]
[rl ha]
[cker,]
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