perl -anle"next unless @L; print if $L[0] eq $F[0]; @L = @F;" in.txt > out.txt

##</code><code>##

perl -anwle 'BEGIN{$L = "";}  print if $F[0] eq $L; $L = $F[0];' in.txt > out.txt

##</code><code>##

perl -anle 'print if $F[0] eq $L; $L = $F[0];' in.txt > out.txt