while ($line=<INFILE>) {
    chomp $line;
    if ($line=~/^COMPND[\s]{3}3/) {
        $line=join(' ',split(' ',$line));       # 1
        chop $line;                             # 2
        $chain=substr($line,length($line)-1,1); # 3
        $lastchain=$chain;                      # 4
        print OUTFILE ("$chain\n");
    } else {
        $lastchain='@';                         # 5
    }
}