$sublen = $num * 2 + 1;

while (<INPUT>) {
    chomp;
    for my $ofs ( 0 .. length()-$sublen )
    {
        my $ngram = substr( $_, $ofs, $sublen );
        next unless $ngram =~ /^.{$num}$graph/;

         # store this ngram to your hash
    }
}