my $f2 = $pos[1] - $pos[0];
my $f3 = $pos[2] - $pos[1];
my $f4 = $pos[3] - $pos[2];
my $f5 = $pos[4] - $pos[3];

foreach $line (@data) {
    my $pictureline = "a$pos[0] a$f2 a$f3 a$f4 a$f5";
    print unpack( $pictureline, $line ), "\n";
}