The typical solution is for the sender to prefix the data with the number of bytes. Then the reader can read (say) a 4 byte integer indicating how many bytes will follow, and then issue a read for the approriate number of bytes, or better, loop reading reasonable size chunks and counting until it has it all:

## sender
my $size = -s 'theFileToSend';
print $socket pack 'N', $size; ## Assumes files less than 4GB.
local $/ = \2**16; ## read the file in 64k chunks
print $socket $_ while <$file>;



## The reader
my $size;
read( $sock, $size, 4 ); ## Get the size in binary
$size = unpack 'N', $size;

while( $size > 0 ) {
    my $buffer;
    my $chunkSize = $size < 2**16 ? $size : 2**16;
    $size -= read( $sock, $buffer, $chunkSize );
    ## do something with this chunk
}
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