sub gen;
sub gen{
    return @_[1..$#_] if $_[0] == 1;
    map{
        my $p=$_;
        map{ $p . $_ } gen( $_[0]-1, @_[1..$#_] )
    } @_[1..$#_]
}

my %seqs = ...;
my @patterns = gen( 7, qw[A C G T] );

my %counts;

for my $seq ( values %seqs ) {
    ++$counts{ substr $seq, $_, 7 } for 0 .. length( $seq )-7;
}

print "$_ ::= $counts{ $_ }" for @patterns;