Use of uninitialized value in string ne at sample.pl line 24, <FH2> line 6.

This refers to

my @keep= grep {$_ < 1 || $num[$_]>=10 || $alp[$_-1] ne $alp[$_-0]} 0..$#num;

This statement appears to be incorrectly constructed. I believe the first argument should be a regex, not a logical conditional.

Revised: Please ignore. As this points out, I was wrong.