use strict;
use warnings;
use feature 'say';

my $headers = qr{
    \A
    \s*
    (?: LOCUS | DEFINITION | ACCESSION | VERSION | KEYWORDS | SOURCE | ORGANISM | CDS )
}x;

my $skip = qr{
    \A
    \s*
    (?: Data\s+file: | \/\/ | \z )
}x;

my @lines;

while (<>) {
    next if /$skip/;
    s/\R//;

    if (/$headers/ and @lines) {
        say @lines;
        @lines = ();
    }

    push @lines, $_;
}

say @lines if @lines;