# DOS quoting:
perl -ne "$contig=$1 if /^Sequence:\s+(\S+)/; print \"$contig $_\" if $_!~/^(\S+:)/ && /\S/" dat.txt
# *nix quoting:
perl -ne '$contig=$1 if /^Sequence:\s+(\S+)/; print "$contig $_" if $_!~/^(\S+:)/ && /\S/' dat.txt