my ($pfmt,$line,@header = ("s50c150");
sub packlen {
    length pack $_[0];
# too fussy    length pack $_[0], (unpack $_[0], (pack $_[0]));
}
read FILE, $line, packlen($pfmt) or die $!;
@header = unpack $pfmt, $line;

##</code><code>##

$ perl -e '$foo="I50a150";print length(pack $foo),$/' 
350
$