open FILE, "<$file" or die $!;
binmode FILE;
my $line;

read FILE, $line, 50*2 + 150*1;
my @header = unpack( "s50c150", $line );
$string = join '', map{ chr($_) } @header[50..149];
# etc, etc...

##</code><code>##

read FILE, $line, sizeof("s50c150");
unpack( "s50c150", $line );