Another way to do it:

open my $FH, '<', './sub/100087.txt' or die "Cannot open './sub/100087
+.txt' because: $!";

my $count = 1;
while ( <$FH> ) {
    next unless /^[a-z]/;
    chomp;
    print;
    print $count++ % 7 && !eof( $FH ) ? ", " : "\n";
    }
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