The most efficient way is using pack and unpack.
This is faster than using substr. Here is a sample chunk of
code to break up and put back together again some fixed-format input lines,
in this case from the output of a normal, Berkeley-style ps:
# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
$PS_T = 'A6 A4 A7 A5 A*';
open(PS, "ps|");
$_ = <PS>; print;
while (<PS>) {
($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
for $var (qw!pid tt stat time command!) {
print "$var: <$$var>\n";
}
print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
"\n";
}